Nitride semiconductor device

ABSTRACT

In the nitride semiconductor device of the present invention, an active layer  12  is sandwiched between a p-type nitride semiconductor layer  11  and an n-type nitride semiconductor layer  13.  The active layer  12  has, at least, a barrier layer  2   a  having an n-type impurity; a well layer  1   a  made of a nitride semiconductor that includes in; and a barrier layer  2   c  that has a p-type impurity, or that has been grown without being doped. An appropriate injection of carriers into the active layer  12  becomes possible by arranging the barrier layer  2   c  nearest to the p-type layer side.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a nitride semiconductor devicewhich uses a nitride semiconductor (In_(X)Al_(Y)Ga_(1-X-Y)N, 0≦X, 0≦Y,X+Y≦1) used in light emitting devices such as light emitting diodedevice (LED) and laser diode device (LD), light receiving devices suchas solar cell and optical sensor or electronic devices such astransistor and power devices, and particularly to a nitridesemiconductor device comprising nitride semiconductor layer whichincludes In.

[0003] 2. Description of the Prior Art

[0004] Recently semiconductor laser devices which use nitridesemiconductor have been receiving increasing demands for theapplications in optical disk systems such as DVD which are capable ofrecording and reproducing a large amount of information with a highdensity. Accordingly, vigorous research efforts are being made in thefield of semiconductor laser device which uses nitride semiconductor.Because of the capability to oscillate and emit visible light over abroad spectrum ranging from ultraviolet to red, the nitridesemiconductor laser, device is expected to have wide applications suchas light sources for laser printer and optical network, as well as thelight source for optical disk systems. The applicant of the presentinvention reported a laser which successfully underwent over tenthousand hours of operation under the conditions of continuousoscillation at a wavelength of 405 nm with output power of 5 mW at theroom temperature.

[0005] Light emitting devices and light receiving devices which usenitride semiconductor have such a structure as a nitride semiconductorwhich includes In-is used for the active layer and, accordingly, it isimportant to form a better active region in the active layer in order toimprove the device characteristics.

[0006] In the prior art, n-type nitride semiconductors doped with n-typeimpurities have beer used for the active layer of the nitridesemiconductor device. Particularly in the case of a device of quantumwell structure, the n-type nitride semiconductors doped with n-typeimpurities have been used in the well layer and, the barrier layer.

[0007] In order for light emitting devices which employ nitridesemiconductors to have applications in wide fields, they must be furtherimproved in the device characteristics, particularly in the devicelifetime.

[0008] It is essential to have a longer lifetime and a higher outputpower for the laser devices which use nitride semiconductors in order tobe used as the light source for reading or writing information inhigh-density optical disk systems described above and have furtherapplications. Other classes of the nitride semiconductor device are alsorequired to have a longer lifetime and a higher output power, and lightemitting devices are required to have a higher output power of lightemission.

[0009] Weak reverse withstanding voltage of the devices using nitridesemiconductor, which has been a problem in the part art, has a highprobability of leading to destruction of the device during handling inthe manufacturing process and mounting on an end product, and istherefore one of the most important problems.

[0010] The present invention has been made in consideration of theproblems described above, and aims at obtaining a nitride semiconductordevice which has excellent device characteristics including thethreshold current density and has longer device lifetime and high outputpower.

SUMMARY OF THE INVENTION

[0011] (1) A light emitting device according to the present invention isa type of nitride semiconductor device having a structure where anactive layer of a quantum well structure, which comprises a well layermade of a nitride semiconductor that includes In, and a barrier layermade of a nitride semiconductor, is sandwiched by a p-type nitridesemiconductor layer and an n-type nitride semiconductor layer, whereinthe light emitting device according to the present invention ischaracterized in that the above active layer has a first barrier layer,that is arranged in a position nearest to the above p-type nitridesemiconductor layer, and a second barrier layer, that is different fromthe first barrier layer, as the above barrier layer and is characterizedin that the above first barrier layer does not substantially include ann-type impurity while the above second barrier layer includes an n-typepurity. Here, though, barrier layers, other than the first barrier layerand the second barrier layer among the barrier layers in the activelayer, are not particularly limited, in the case of usage as a laserdevice or as a light emitting device of high power, they are preferablydoped with an n-type impurity or are not doped with any impurities.

[0012] Though, in a conventional multiple quantum well-type (hereinafterreferred to as MQW-type). nitride semiconductor device, all the barrierlayers are, in general, doped with an n-type impurity, such as Si, inorder to enhance light emission efficiency by increasing the initialelectron concentration in the active layer, a nitride semiconductordevice of the present invention has a barrier layer, that is doped withan n-type impurity in the same manner as in the prior art, while ann-type impurity is not substantially included only in the first barrierlayer that is nearest to the p-type nitride semiconductor layer. In sucha structure, characteristics with respect to the device lifetime and thereverse withstanding voltage of the nitride semiconductor device can beimproved.

[0013] Though the mechanism where the lifetime characteristic isimproved is not necessarily evident, it can be inferred that, for onereason, the fact that the lifetime of the carriers has become longerthan in the prior art contributes to this mechanism. Conventionally abarrier layer, that is doped with an n-type impurity, is arranged on theside of the p-type layer so that diffusion of the p-type impurity fromthe p-type layer occurs to quite a great degree and, thereby, a barrierlayer, that includes an n-type impurity and a p-type impurity, isprovided, which causes the lowering of the lifetime of the carriers.According to the present invention, since the first barrier layer is notdoped with an n-type impurity, n-type and p-type impurities can beprevented from coexisting in the same barrier layer.

[0014] In addition, among barrier layers in the active layer, thebarrier layer arranged on the side of the p-type layer (first barrierlayer) does not substantially include an n-type impurity so as to have afunction different from that of the barrier layer (second barrierlayer), which has an n-type impurity, in the active layer. That is tosay, by having the second barrier layer, the carriers injected from then-type layer into the active layer are increased and the carriers thatreach deep into the active layer (to the p-type layer side) areincreased so that the injection efficiency of the carriers can beincreased while, by having the first barrier layer, a barrier layer, inwhich an n-type impurity is not included, is arranged as a barrier layernearest to the p-type layer in the active layer so that it becomespossible to increase the injection of the carriers from the p-type layerand also to improve the efficiency.

[0015] In the case that an n-type impurity is included in the firstbarrier layer, the injection of the carriers from the p-type layer tendsto be blocked. In particular, the diffusion distance of the carriersfrom the p-type layer tends to be short in comparison with the carriersfrom the n-type layer and, therefore, when the first barrier layer,which corresponds to the entrance for the injection of the carriers fromthe p-type layer to the active layer, has an n-type impurity, theinjection of the carriers from the p-type layer is negatively affectedto a serious degree. As shown in FIG. 14, it is understood that thedevice lifetime is suddenly lowered as the n-type impurity concentrationin the first barrier layer is increased.

[0016] Accordingly, by providing the first barrier layer in the activelayer, it is observed that a great number of holes can be provided andthe lifetime of the carriers tends to become longer such that they areconsidered to contribute to the increase of the above characteristics.

[0017] Though the second barrier layer may adjoin the first barrierlayer, it is preferably provided at a distance away from the firstbarrier layer by making at least one, or more, well layer intervene.Thereby, the first barrier layer arranged on the p side and the secondbarrier layer arranged on the n side are provided with a well layerplaced between them within the active layer so that an effective carrierinjection becomes possible so as to reduce the loss in a laser device asa light source for, for example an optical disk system, and the devicecharacteristics, in particular the device lifetime and the power, aresubsequently increased. At this time, the second barrier layer ispreferably a barrier layer nearest to the n-type layer among the barrierlayers in the active layer so as to be the entrance for the injection ofthe carriers from the n-type layer so that a great amount of carrierinjection or an effective injection becomes possible and the devicecharacteristics are improved.

[0018] Here, the fact that an n-type impurity is not substantiallyincluded indicates that an n-type impurity is not included due to theexceeding of the concentration resulting from the contamination, or thelike, during the process and, for example, in the case that the n-typeimpurity is Si, the fact indicates that the concentration is 5×1016cm⁻³, or less.

[0019] (2) It is preferable for the film thickness of the above firstbarrier layer to be greater than the film thickness of the secondbarrier layer. In this structure increase of the device lifetime can beimplemented. In the case that the first barrier layer has a filmthickness less than that of the other barrier layer (second barrierlayer), lowering of the device lifetime can be observed. In particular,this tendency is significant in the case that the first barrier layer isarranged in the outermost position. In addition, in the case that thefirst barrier layer is positioned in the outermost position in theactive layer, that is to say, on the top, when a p-type nitridesemiconductor layer is provided on the active layer, the reduction ofthe above device lifetime is furthered. For example, as shown in FIG. 8,in the case that the first harrier layer 2 c is arranged nearest to thep-type electron confining layer (first p-type nitride semiconductorlayer), the first barrier layer becomes an important layer where thefilm Thickness thereof determines the characteristics of the activelayer and the well layer since this p-type electron confining layer is alayer that strongly affects the active layer, particularly the welllayer, as described below.

[0020] That is to say, in the nitride semiconductor device according tothe present invention, carriers can be effectively confined in theactive layer when the barrier layers in the active layer are formed of anitride semiconductor that includes In and the layer, at least,adjoining the active layer among p-type nitride semiconductor layers isformed of a nitride semiconductor (electrons confining layer) thatincludes Al. However, when a nitride semiconductor that includes Al ismade to grow after a nitride semiconductor that includes In is made togrow, the nitride semiconductor that includes In is easiy resolvedbecause of the high vapor pressure of InN and because of the differencein the growth conditions of these nitride semiconductors. Therefore, itis preferable for the first barrier layer to be formed thicker than theother barrier layers.

[0021] For example, in the case of the growth by means of an MCCVDmethod, it is general that InGaN is made to grow under the conditions ofa slow gas flow rate at a low temperature in a nitrogen gas atmospherewhile AlGaN is made to grow under the conditions of a fast gas flow rateat a high temperature inahydrogen gas atmosphere. Accordingly, forexample, when, after growing InGaN as, the first barrier layer, AlGaN ismade to grow as a p-type nitride semiconductor layer, InGaN is resolvedthrough a gas etching at the time when the growth condition within thereaction vessel is switched to another condition. Therefore, by formingthe first barrier layer thicker than the other barrier layers, anexcellent quantum well structure can be maintained even in the case thatthe first barrier layer is slightly resolved. That is to say, the firstbarrier layer plays the role of a protective layer that prevents theactive layer, which includes In, from being resolved.

[0022] Furthermore, in the case that the first barrier layer arrangednearest to the p-type nitride semiconductor layer has a great filmthickness, the distance vis-à-vis the p-type electron confining layercan be increased so that carriers of a high concentration can be stablyinjected in the continuous drive of the device since a sufficientlybroad space can be secured even for a great amount of p-type carriers.Accordingly, device reliability, such as a long device lifetime, can beimproved.

[0023] (3) In addition, when the barrier layer arranged in the positionnearest to the n-type nitride semiconductor layer is assumed to be abarrier layer B1 and the i-th (i=1, 2, 3 . . .L) barrier layer countedfrom the barrier layer B1 toward the above p-type nitride semiconductorlayer is assumed to be a barrier layer B1, it is preferable for barrierlayers Bi from i=1 to i=n (1<n<L) to have an n-type impurity. Because ofthis structure the injection of the carriers to each well layer in theactive layer becomes more efficient. In addition, the injection of thecarriers deep into the active layer (p-type layer side) is effectivelycarried out so that the device can deal with a great amount of carrierinjection. Accordingly, the light emission efficiency is improved, forexample, in an LED or in a LD and it becomes possible to lower theoscillation threshold current density and the forward direction voltagewhile increasing the device lifetime. In addition, the provision of ann-type impurity in the barrier layers bi from the first to the n-thcontributes to the lowering of the threshold current density because thecarriers are immediately injected into the well layers at the initialphase of the drive of the device.

[0024] (4) In addition, all of the barrier layers, other than the firstbarrier layer, are preferably doped with an n-type impurity. Thereby,the carrier injection from the n-type layers can further be increasedand can be made more effective.

[0025] (5) It is preferable for the above first barrier layer to bearranged in the outermost position of the above active layer. The firstbarrier layer is arranged on the side nearest to the p-type nitridesemiconductor layer within the active layer so that the first barrierlayer becomes the entrance for the injection of the carriers and,thereby, the injection of the carriers from the p-type layer to theactive layer becomes effective and a great amount of carriers can beinjected so as to improve device characteristics, such as the thresholdcurrent density, the device lifetime and power. In addition, a nitridesemiconductor device can be gained which has the device reliability thatcan withstand severe drive conditions, such as a great amount of currentor a high power. At this time, it is preferable for the p-type nitridesemiconductor layer to be formed so as to contact the active layer andthe below described first p-type nitride semiconductor layer can beprovided as a layer that contacts the first barrier layer.

[0026] (6) Furthermore, it is preferable for the above second barrierlayer to be arranged in the outermost position close to the above n-typenitride semiconductor layer within the above active layer. In thisstructure, the active layer is provided wherein the first p side barrierlayer and the second n side barrier layer are respectively arranged onthe p-type nitride semiconductor layer side and on the n-type nitridesemiconductor layer side so that the carriers from the p-type layer andn-type layer are effectively injected toward the center portion of theactive layer.

[0027] (7) It is preferable in the above structure (6) for the filmthickness of the above first p side barrier layer to be approximatelythe same as the film thickness of the above second n side barrier layer.In this structure, the active layer becomes more symmetrical and, as aresult, the dispersion of the devices can be restrained so as toincrease the yield and the threshold current density is reduced.

[0028] (8) In addition, it is preferable in the above structure (6) forthe above active layer to have two, or more, well layers so as to have athird barrier layer between these well layers and it is also preferablefor the film thickness of the above third barrier layer to be less thanthe film thicknesses of the above first p side barrier layer and of thesecond n side barrier layer. In this structure, it becomes possible forthe second n side barrier layer and the first p side barrier layer, aswell as the third barrier layer, to have different functions so that itbecomes possible to restrain the dispersion of the devicecharacteristics and to reduce the threshold current density Vf. That isto say, the second n side barrier layer and the first p side barrierlayer are arranged in the outermost position in the active layer so asto be the entrances for the injections of the carriers from the n-typelayer and p-type layer while the film thickness is greater than thethird barrier layer so that a broad space for holding a great amount ofcarriers Is secured and, contrarily, the film thickness of the thirdbarrier layer is small so that the film thickness of the entirety of theactive layer can be reduced so as to contribute to the reduction of Vf.

[0029] (9) It is preferable for at least one well layer within the aboveactive layer to have a film thickness of 40 Å, or more. Conventionally,the film thickness of the well layer is regarded as optimal in apreferable range of from approximately 20 Å to 30 Å since thecharacteristics (for example, oscillation threshold current) at theinitial stage of oscillation and the light emission are taken intoconsideration which results in the fact that a continuous drive with agreat current accelerates the device deterioration and prevents theincrease of the device lifetime. The present invention solves thisproblem due to the above structure.

[0030] That is to say, the structure of the present invention makes aneffective carrier injection possible and, additionally, by providing awell layer of which the film thickness is suitable for the carrierinjection, it becomes possible to increase the stability in the drive ofa light emitting device and a laser device of high power and loss inoutput, relative to the injected current, can be reduced so that a greatincrease in the device lifetime can be made possible. An effective lightemitting recombination, without loss, of the great amount of carriersinjected in the well layer is required for light emission andoscillation at high power and the above structure is suitable forimplementing such light emission and oscillation.

[0031] The upper limit of the film thickness of the well layer dependson the film thicknesses of the barrier layers and of the active layerand is preferably 500 Å, or less, though it is not particularly limitedto this. In particular, it is preferably 300 Å, or less, when it istaken into consideration that a plurality of layers are layered in thequantum well structure. Furthermore, in the case that the film thicknessof a well layer is in the range of no less than 50 Å and no more than200 Å, it is possible to form, preferably, an active layer in either amultiple quantum well structure or in a single quantum well structure.In the case of the multiple quantum well structure in particular, thefilm thickness of a well layer is preferably in the range of no lessthan 50 Å and no more than 200 Å, since the number of layers (number ofpairs of a well layer and a barrier layer) is increased. In addition,when the film thickness of a well layer is in this preferable range, ahigh reliability of the device and a long lifetime can be gained forlight emission and oscillation with a large amount of current and with ahigh power output while, in a laser device, a continuous oscillation at80 mW becomes possible and an excellent device lifetime can beimplemented in a broad output range such as from 5 mW to 80 mW. At thistime, it is necessary to adopt the above range of film thickness of awell layer for at least one well layer in the case that the active layerhas a multiple quantum well structure and preferably the above filmthicknesses are adopted for all of the well layers. By doing so, theabove described effects are gained in each of the well layers so that alight emitting recombination and a photoelectric conversion efficiencyare further improved. By using a nitride semiconductor that includes In,more preferably InGaN, for a well layer, an excellent device lifetimecan be gained. At this time, by making the composition ratio x of In inthe range of 0<x≦0.3, a wel layer of a thick film with a good crystalcan be formed and preferably by making x≦0.2, a plurality of well layersof thick films with a good crystal structure can be formed so that anactive layer in a good MQW structure can be gained.

[0032] (10) The above described first barrier layer preferably has ap-type impurity. In this structure, the above described injection ofcarriers from the p-type layer becomes effective and the lifetime of thecarriers tends to increase and, as a result, the structure contributesto increases in the reverse withstanding voltage, the device lifetimeand the output. This is because, as described above, a carrier injectionfrom the p-type layer becomes excellent due to substantially noinclusion of an n-type impurity and, furthermore, it becomes possible toaccelerate further injection of carriers into the active layer by havinga p-type impurity in the first barrier Layer so that a large amount ofcarriers are effectively injected from the p-type layer into the activelayer or deep inside the active layer (n-type layer side) and, thereby,increases in light emitting recombination, photoelectric conversionefficiency and device lifetime and, in addition, an improvement in thecharacteristic of reverse withstanding can be implemented.

[0033] (11) In addition, though the concentration of the p-type impurityin the first barrier layer is not in particular limited, it ispreferable to be no less than 1×1016 cm−3 and no more than 1×1019 cm−3.In the case the p-type impurity concentration is too low, the holeinjection efficiency into the well layer is lowered, while if it is toohigh, the carrier mobility in the first barrier layer is lowered so asto increase the Vf value of the laser.

[0034] (12) The first barrier layer of which the p-type impurityconcentration is in such a range is an i-type or a p-type.

[0035] (13) The doping of a p-type impurity into the first barrier layeris preferably carried out through diffusion from the p-type nitridesemiconductor layer after making the undoped first barrier layer growrather than being carried out at the time of the growth of the firstbarrier layer. This is because, in the case that it is carried out atthe time of the growth of the first barrier layer, a p-type impuritydiffuses into an n-type well layer beneath the first barrier layer atthe time when the first barrier layer grows so that the device lifetimecharacteristics is lowered, on the other hand, in the case that thedoping of a p-type impurity is carried out through diffusion, a p-typeimpurity can be doped into the first barrier layer without affecting thewell layer.

[0036] (14) In the case that an n-type nitride semiconductor layer, anactive layer and a p-type nitride semiconductor layer are layered insequence in the device structure, the barrier layer can have a p-typeimpurity because a p-type impurity diffuses from the p-type nitridesemiconductor layer that is made to grow subsequent to the first barrierlayer, which is made to grow without doping.

[0037] (15) A nitride semiconductor device of the present inventionpreferably has a laser device structure wherein said p-type nitridesemiconductor layer has an upper clad layer made of a nitridesemiconductor that includes Al of which the average mixed crystal ratioof x, wherein 0<x≦0.05 and wherein said n type nitride semiconductorlayer has a lower clad layer made of a nitride semiconductor thatincludes Al of which the average mixed crystal ratio of x, wherein0<x≦0.05. A laser device gained in such a structure can continuouslyoscillate with the output of 5 mW to 100 mW so as to become an LD havingdevice characteristics suitable for a reading and writing light sourcein an optical disk system and makes it possible to implement a longlifetime. By limiting the average mixed crystal ratio of Al in the cladlayer to 0.05, or less, an optical wave guide which makes it possible tocontrol a self-exciting oscillation at the time of a high power outputis provided so that a continuous oscillation with a high power output ina stable manner becomes possible and it also becomes possible to gain anLD for an optical disk light source. Though, conventionally, a nitridesemiconductor of which the average composition of Al in the clad layeris no less than 0.05 and no more than 0.3 is used, in this structureconfinement of light becomes too strong and, thereby, a self-excitingoscillation is generated in a continuous oscillation with a high outputof 30 mW, or more. This self-exciting oscillation is due to thegeneration of a kink in the current-light output characteristics that isgenerated on the low output side in an LD structure which has a stronglight confinement in the longitudinal direction so as to enhance thelight density and such generation of a kink thus becomes disadvantageousas a light source of an optical disk system so that a self-excitingoscillation due to the kink is unstable and leads to dispersion in thedevices. According to the structure of the present invention, an opticalwave guide, of which the refraction difference in a clad layer isreduced, is gained and by using an active layer that is in the abovedescribed range, a large amount of carriers are continuously injected ina stable manner for light emitting recombination in the structure sothat a continuous oscillation can be gained so as to exceed thecompensation for the loss due to the lowering of the light confinementin the clad layer and, moreover, the light emission efficiency withinthe active layer can be enhanced.

[0038] It is preferable that said upper clad layer has a p-typeconductivity and said lower clad layer has an n-type conductivity, andthat said active layer has a first barrier layer that is arranged in aposition nearest to said upper clad layer as said barrier layer and asecond barrier layer that is different from the first barrier layer and,at the same time, it is preferable that said first barrier layer has ap-type impurity and said second barrier layer has an n-type impurity. Insuch a structure, as described above, injection of carriers from thep-type layer is carried out in an excellent condition and, as a result,device characteristics, in particular device lifetime, are improved.

[0039] (16) The above p-type nitride semiconductor layer preferablycontains a first p-type nitride semiconductor layer so as to adjoin theactive layer so that the first p-type nitride semiconductor layer ismade of a nitride semiconductor that includes Al. In this structure, asshown in FIGS. 4 to 7, the first p-type nitride semiconductor layer 28functions as an electron confining layer and, in particular, makes itpossible to confine a large amount of carriers within the active layerin an LD and an LE with a large current drive for high power output. Inaddition, in the relationships between the above first barrier layer,barrier layer BL and the first p side barrier layer, as shown in FIG. 8,the film thicknesses of these barrier layers determine the distance dBbetween the first p-type nitride semiconductor layer and a well layer 1b so as to greatly affect the device characteristics.

[0040] In addition, since the first p-type nitride semiconductor layermay grow in the form of a thin film, it can be made to grow at atemperature lower than that for a p-type clad layer. Accordingly, byforming a p-type electron confining layer, resolution of an active layerthat includes In can be prevented as opposed to the case where a p-typeclad layer is directly formed on the active layer. That is to say, thep-type electron confining layer plays the roles of preventing theresolution of the active that includes In in the same manner as does thebarrier layer of FIG. 1.

[0041] (17) The above described first p-type nitride semiconductor layeris provided so as to contact the barrier layer nearest to the abovedescribed p-type nitride semiconductor layer, and, preferably, is formedof a semiconductor which has been grown by being doped with a p-typeimpurity of which the concentration is higher than that in a barrierlayer in the above active layer. Because of this structure, theinjection of carriers from the p-type layer to the barrier layer (theabove described first barrier layer) closest to the p-type layer becomeseasy to implement. In addition, by doping a p-type impurity of a highconcentration into the first p-type nitride oxide layer, a p-typeimpurity is diffused into the barrier layer so that an appropriatep-type impurity can be added. Thus, since it becomes unnecessary to addan impurity at the time of barrier layer growth, it becomes possible tomake a barrier layer grow with a good crystal structure. In particular,in the case that this barrier layer is a nitride semiconductor thatincludes In, crystal deterioration is great due to impurity addition andthe effect thereof is significant. In addition, in the case that thefirst p-type nitride semiconductor layer is, as described below, anitride semiconductor that includes Al and the Al mixed crystal ratiothereof is higher than the mixed crystal ratio of Al in the p-type cladlayer, it effectively functions as an electron confining layer whichconfines electrons within the active layer and the effects are gainedwherein an oscillation threshold value and driving current are loweredin a large current drive, a high power LD and an LED.

[0042] (18) It is preferable for the number of well layers to be in therange of from no less than 1 to no more than 3 in the above describedactive layers. In this structure it becomes possible to lower theoscillation threshold value in comparison with the case where the numberof threshold layers is 4, or more. In addition, at this time by makingthe film thickness of the well layer 40 Å, or more, as described above,a broad space is secured inside of a small number of well layers so thatan effective light emitting recombination becomes possible even in thecase that a large amount of carriers are injected so that this makes anincrease in the device lifetime and an increase in the light emissionoutput possible. In particular, in the case that the film thickness ofthe well layer is 40 Å, or less, and the number of well layers is 4, ormore, a large amount of carriers are infected into each well layer of athin film, in comparison with the above described case, in order to gaina high power LD or LED by driving the well layers with a large currentso that the well layers are driven under severe conditions and devicedeterioration occurs at an early stage. In addition, when the number ofwell layers increases, the carriers are not distributed uniformly but,rather, tend to be distributed unevenly so that the above describeddevice deterioration becomes a critical problem in the case wherein thedevice is driven with a large amount of current under such a condition.In such a structure as described above, the barrier layer that isnearest to the p-type layer side does not include an n-type impurity orhave a p-type impurity, or other barrier layers each do have an n-typeimpurity and, thereby, a large amount of carriers can be injected intothe well layers in a stable manner and, in addition, the well layer ismaintained at the above described film thickness (40 Å, or more) so thatthese closely relate to each other to appropriately work to implement anexcellent device lifetime and a high light emission output in aconsecutive driving of the device.

[0043] (19) It is preferable that the above described second barrierlayer is arranged so as to be sandwiched by well layers and, the filmthickness ratio Rt of the above described well layer to the secondbarrier layer is in the range of from 0.5≦Rt≦3 Because of this structurea light emitting device and a laser device can be gained wherein theresponse characteristics are excellent and RIN is low in order to beused specifically for an optical disk system, an optical communicationsystem, and the like. That is to say, though the film thicknesses of thewell layers, the barrier layers and the active layers become factorsthat greatly affect the RIN and the response characteristics in theactive layer of a quantum well structure, a light emitting device and alaser device that are excellent in these characteristics can be gainedby limiting the film thickness ratio of the well layer to the barrierlayer to the above described range in this structure.

[0044] (20) It is preferable for the film thickness dw of the abovedescribed well layer to be in the range of 40 Å≦dw≦100 Å and for thefilm thickness db of the above described second barrier layer to be inthe range of db≧40 Å. In this structure, by adjusting the film thicknessof the well layer so that the above described film thickness ratio Rt isin the above described range, a laser device having a long lifetime, ahigh power output, as shown in FIG. 12, and having RIN characteristicsas well as response characteristics that are suitable for the lightsource of an optical disk system. That is to say, in the light emittingdevice of the present invention the lifetime can be made longer byincreasing the film thickness of the well layer while, on the otherhand, the response characteristics and the RIN characteristics tend tobe lowered when the film thickness of the well layer is increased. Inthis structure, this is appropriately improved. In addition, in the casethat the film thickness of the barrier layer is 40 Å, or more, anexcellent device lifetime can be gained so that a laser device thatbecomes an-excellent light source in an optical disk system can begained as shown in FIG. 13.

[0045] (21) It is preferable for the above described p-type nitridesemiconductor layer and the above described n-type nitride semiconductorlayer to have, respectively, an upper clad layer and a lower clad layerso that the average mixed crystal ratio of Al in the nitridesemiconductor of the upper clad layers become greater than that in thelower clad layers. This is because, in an effective refraction typelaser device, confinement in the lateral direction can be reduced byincreasing the mixed crystal ratio of Al in the upper clad layer,wherein an effective refraction difference is created, so as to have anupper clad layer of which the refraction is small. That is to say, byreducing the refraction difference between a buried layer, which createsan effective refraction difference on both sides of the wave guide, andthe upper clad layer, the structure can be gained wherein theconfinement in the lateral direction is made smaller. Thus, theconfinement in the lateral direction is reduced and light density isreduced and, thereby, a laser device can be gained wherein, up to a highoutput range, no kink is generated.

[0046] (22) Furthermore, the average mixed crystal ratio x of Al in theabove described upper clad layer is in the range of 0<x≦0.1 and,thereby, a laser device can be gained which has the lasercharacteristics, in particular, characteristics such as current-lightoutput characteristics, that can be used appropriately for an opticaldisk system. At this time the oscillation wavelength of the laser devicecan be adjusted in the range of from no less than 380 nm to no more than420 nm so that an appropriate laser device can be gained by using theabove described clad layer.

[0047] (23) The above described p-type nitride semiconductor layer has afirst p-type nitride semiconductor layer that contacts the abovedescribed active layer and becomes an electron confining layer and theactive layer has a well layer of which the distance dB from the firstp-type nitride semiconductor layer is in the range of from no less than100 Å to no more than 400 Å and has a first barrier layer within thedistance dB and, thereby, a nitride semiconductor device of which thedevice lifetime is excellent can be gained. This can be made to be adevice structure wherein, as shown in FIG. 8, the distance dB from thefirst p-type nitride semiconductor layer 28 has a first barrier layer,that is to say, a barrier layer that does not substantially have ann-type impurity or that is adjusted to have a p-type impurity and,thereby, device deterioration due to the first p-type nitridesemiconductor layer, which is a p-type carrier confining layer, isprevented so as to improve the device lifetime and it becomes possibleto accelerate a light emitting recombination in a well layer arrangedoutside of the distance dB. Here, the device has, at least, the abovedescribed first barrier layer in the region of the distance dB, that isto say, the device has an impurity adjusted region, wherein, asdescribed above, an impurity, or the amount of the impurity, is adjustedin at least a portion thereof in the region of the distance dB. At thistime, the distance dB is preferably the first harrier layer, that is tosay, a first barrier layer of which the film thickness is dB ispreferably formed so as to contact the first p-type nitridesemiconductor layer so that the above described effects can be maximallygained. In this manner, by using the region of the distance dB as animpurity adjusted region, as shown in FIG. 8B, a structure can be gainedwherein a plurality of layers of different band gap energies areprovided. For example, in FIG. 8B a region 4, of which the band gapenergy is smaller than that of the barrier layer 2 c, is formed whereinthe above described effects can be gained by making the region dB animpurity adjusted region. Contrarily, in a similar manner, a layer 4,which has band gap energy larger than that of the barrier layer 2 b, maybe provided. That is to say, in the case that a plurality of layers ofwhich the band gap energies are different are provided in the region dB,a device of which the characteristics are excellent can be gained byadjusting the impurity, or the amount of the impurity, in the region dBwhich is used as the first barrier layer. Furthermore, the distance dBis preferably in the range of from no less than 120 Å to no more than200 Å so that a nitride semiconductor device with an appropriate activelayer in the device structure can be gained.

[0048] As the n-type impurity used in the nitride semiconductor deviceof the present invention, group IV or VI elements such as Si, Ge, Sn, S,O, Ti and Zr may be used, while Si, Ge or Sn is preferable and mostpreferably Si is used. As the p-type impurity, Be, Zn, Mn, Cr, Mg, Ca orthe like may be used, and Mg is preferably used.

[0049] For the purpose of the present invention, the term undoped meansa nitride semiconductor grown without adding p-type impurity or n-typeimpurity as a dopant, for example organometallic vapor phase growingprocess in a reaction vessel without supplying any impurity which wouldserve as dopant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050]FIG. 1 is a schematic sectional view showing one embodiment of thepresent invention.

[0051]FIG. 2 is a schematic sectional view showing one embodiment of thepresent invention.

[0052]FIG. 3 is a schematic sectional view showing one embodiment of thepresent invention.

[0053]FIG. 4 is a schematic sectional view of stacked structure andschematic diagram showing band structure according to one embodiment ofthe present invention.

[0054]FIG. 5 is a schematic sectional view of stacked structure andschematic diagram showing band structure according to one embodiment ofthe present invention.

[0055]FIG. 6 is a schematic sectional view of stacked structure and aschematic diagram showing band structure according to one embodiment ofthe present invention.

[0056]FIG. 7 is a schematic sectional view of stacked structure and aschematic diagram showing band structure according to one embodiment ofthe present invention.

[0057]FIGS. 8A and 8B is a schematic sectional view of stacked structureand schematic diagram showing band structure according to one embodimentof the present invention.

[0058]FIGS. 9A and 9B is a schematic sectional view of a deviceaccording to one embodiment of the present invention.

[0059]FIG. 10 is a schematic sectional view of stacked structure andschematic diagram showing band structure according to one embodiment ofthe present invention.

[0060]FIG. 11 is a schematic sectional view showing one embodiment ofthe present invention.

[0061]FIG. 12 is a diagram showing the relationship between the devicelifetime and well layer thickness in one embodiment of the presentinvention.

[0062]FIG. 13 is a diagram showing the relationship between the devicelifetime and barrier layer thickness in one embodiment of the presentinvention.

[0063]FIG. 14 is a diagram showing the relationship between the devicelifetime and doping concentration in one embodiment of the presentinvention.

[0064]FIG. 15 is a diagram showing the relationship between the reversewithstanding voltage and doping concentration in one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] The nitride semiconductor used in the nitride semiconductordevice of the present invention may be GaN, AlN or InN, or a mixedcrystal thereof, namely gallium nitride compound semiconductor(In_(x)Al_(y)Ga_(1-x-y)N, 0≦x, 0≦y, x+y≦1). Mixed crystals made by usingB as the group III element or by partially replacing N of the group Velement with P or As may also be used.

[0066] (Active Layer)

[0067] The active layer of the present invention has quantum wellstructure which may be either multiple quantum well structure or singlequantum well structure, but Preferably multiple quantum well structurewhich makes it possible to increase the output power and decrease thethreshold of oscillation. The quantum well structure of the active layermay be constituted from well layers and barrier layers to be describedlater being stacked one on another. This structure of stacked layers issuch that the well layer is sandwiched by the barrier layers. In thecase of single quantum well structure, at least one barrier layer isprovided each on the p-type nitride semiconductor layer side and on then-type nitride semiconductor layer side so as to sandwich the welllayer. In the case of multiple quantum well structure, the active layercomprising a plurality of well layers and barrier layers stacked one onanother is constituted as described later in preferred embodiments.

[0068] The active layer has preferably such a structure as barrierlayers are provided as layers located at positions nearest to the n-typenitride semiconductor layer and the p-type nitride semiconductor layer(hereinafter referred to as outermost layer). The outermost layerslocated on both sides are preferably barrier layers.

[0069] In the multiple quantum well structure, the barrier layersandwiched between the well layers is not limited to a single layer(well layer/barrier layer/well layer), and two or more barrier layers ofdifferent compositions and/or impurity concentrations may be stackedsuch as well layer/barrier layer (1)/barrier layer (2) . . . /welllayer. For example, such a structure may also be employed as an upperbarrier layer 403 made of a nitride semiconductor including Al and alower barrier layer 402 having an energy band gap smaller than that ofthe upper barrier layer are provided between the well layers 401, asshown in FIG. 10.

[0070] (Well Layer)

[0071] The well layer of the present invention is preferably made of anitride semiconductor including In, having a composition of(In_(α)Ga_(1-α)N (0<α≦1). This constitution makes the well layer capableof oscillating and emitting light satisfactorily. Wavelength of theemitted light can be determined by controlling the proportion of In. Inaddition to InGaN, the nitride semiconductors described above such asInAlGaN and InN may be used and nitride semiconductors which do notinclude In may also be used in the present invention, but a nitridesemiconductors which includes In as higher efficiency of light emissionand is more preferable.

[0072] Thickness and number of the well layers may be determined asrequired except for the case to be described later in the fifthembodiment. Specifically, thickness of the well layer is in a range from10 Å to 300 Å, and preferably in a range from 20 Å to 200 Å, whichallows it to decrease Vf and the threshold current density. When thecrystal growth is taken into consideration a layer of relativelyhomogeneous quality without significant variations in the thickness canbe obtained when the thickness is 20 Å or greater, and the crystal canbe grown while minimizing the generation of crystal defects by limitingthe thickness within 200 Å. There is no limitation on the number of thewell layers provided in the active layer, which may be 1 or more. Whenfour or more well layers with larger thickness of layers constitutingthe active layer, total thickness of the active layers becomes too largeand the value of Vf increases. Therefore, it is desirable to restrictthe thickness of the well layer within 100 Å thereby to restrain thethickness of the active layer.

[0073] The well layer may bc doped or undoped with n-type impurity. Whena nitride semiconductor which includes In is used as the well layer,however, an increase in the concentration of r-type impurity leads tolower crystallinity and therefore it is preferable to restrict theconcentration of the n-type impurity thereby to make the well layer ofgood crystallinity. Specifically, in order to achieve bestcrystallinity, the well layer is preferably grown without doping, withthe n-type impurity concentration being kept within 5×10¹⁶/cm³. When thewell layer is doped with n-type impurity, controlling the n-typeimpurity concentration within a range from 1×10¹⁸/cm³ to 5×10¹⁶/cm³makes it possible to suppress the degradation of crystallinity andincreases the carrier concentration, thereby decreasing the thresholdcurrent density and the value of Vf. At this time, it is preferable tokeep the n-type impurity concentration in the well layers nearly equalto or less than the n-type impurity concentration in the barrier layer,which will accelerate the light emission recombination in the well layerand increase the light emission output power. Thus the well layers dopedwith the n-type impurity are preferably used in a low output device suchas LD or LED having output power of 5 mW, having effects of decreasingthe value of Vf and the threshold current density. In order to keep then-type impurity concentration in the well layers nearly equal to or lessthan that of the barrier layer, the well layer may be doped with largeramount of n-type impurity than that for the barrier layer when growing,or modified doping may be employed wherein the barrier layer is grownwhile being doped and the well layers are grown without doping. At thistime, the well layers and the barrier layers may be grown without dopingthereby to constitute a part of the active layer.

[0074] In a device driven with a large current (such as LD or LED ofhigh output power), recombination of the carriers in the well layers isaccelerated and light emission recombination occurs at a higherprobability when the well layers are undoped and do not substantiallyinclude n-type impurity. When the well layers are doped with the n-typeimpurity, in contrast, carrier concentration in the well layersincreases resulting in lower probability of the light emissionrecombination which leads to a vicious cycle of the drive currentincreasing with a constant output power, and consequently causing thedevice reliability (lifetime of the device) to decrease significantly.For this reason, in a high power device (such as LD or LED of outputpower in a range from 5 to 100 mW), the n-type impurity concentration inthe well layers is kept not higher than 1×10¹⁸/cm³, and preferably thewell layers are grown without doping or with such a concentration thatcan be regarded as including substantially no n-type impurity, whichmakes it possible to achieve a nitride semiconductor device capable ofstable operation with high output power. In a laser device with the welllayer being doped with n-type impurity, spectrum width of the peak ofthe laser beam tends to spread and therefore the n-type impurityconcentration is kept within 1×10¹⁸/cm³, and preferably 1×10¹⁷/cm³.Doping n-type impurity in the well layer also degrades the crystallinityof the well layer. If a device with the low-crystallinity well layer isoperated with so high current density as likely in a laser device, thewell and device is degraded and the life of the device is shorten.

[0075] (Barrier Layer)

[0076] According to the present invention, there is no limitation to thecomposition of the barrier layer, and a nitride semiconductor includingIn with the proportion of In being lower than that of the well layer, ora nitride semiconductor including GaN or Al may be used. Specificcomposition may be In_(β)Al_(γ)Ga_(1-β-γ)N (0≦β≦1, 0≦γ<1),In_(β)Ga_(1-β)N (0≦β<1, α>β, GaN or Al_(γ)Ga_(1-γ)N (0<γ<1) In the caseof a barrier layer (lower barrier layer) which serves as the base layerin contact with the well layer, a nitride semiconductor which does notinclude Al is preferably used. Specifically, In_(β)Ga_(1-β)N (0≦β<1,α>β) or GaN is preferably used. This is because growing a well layermade of a nitride semiconductor which includes In directly on a nitridesemiconductor which includes Al such as AlGaN leads to lowercrystallinity, eventually resulting in impeded function of the welllayer Also it is intended to make the band gap energy of the barrierlayer higher than that of the well layer. Best combination of thecompositions of the well layers and the barrier layers may be selectedfrom the compositions described above, by giving consideration to thefact that the barrier layer serves also as the base layer whichdetermines the crystallinity of the well layer.

[0077] The barrier layer may be doped or undoped with the n-typeimpurity, except for the barrier layer located nearest to the p-typelayer to be described later, but preferably doped with the n-typeimpurity. When doped, the n-type impurity concentration in the barrierlayer is preferably 5×10¹⁶/c³ or higher and lower than 1×10²⁰/cm³. Inthe case of LED which is not required to have a high output power, forexample, the n-type impurity concentration is preferably in a range from5×10¹⁶/cm³ to 2×10¹⁸/cm³. For LED of higher output power and LD, it ispreferable to dope in a range from 5×10¹⁷/cm³ to 1×10²⁰/cm³ and morepreferably in a range from 1×10¹⁸/cm³ to 5×10¹⁹/cm³. When doping to sucha high concentration, it is preferable to grow the well layer withoutdoping or with substantially no n-type impurity included. The reason forthe n-type impurity concentration being different among the regular LED,the high-power LED and the high-power LD (output power in a range from 5to 100 nW) is that a device of high output power requires higher carrierconcentration in order to drive with larger current for higher outputpower. Doping in the range described above, as described above, it ismade possible to inject the carrier to a high concentration with goodcrystallinity. In the case of a nitride semiconductor device such aslower-power LD, LED or the like, in contrast, a part of the barrierlayer of the active layer may be doped with the n-type impurity or theentire barrier layers maybe formed with substantially no n-type impurityincluded. When doping with the n-type impurity, all the barrier layersof the active layer may be doped or a part of the barrier layers may bedoped.

[0078] While there is no limitation to the thickness of the barrierlayer, the thickness is preferably not larger than 500 Å, and morespecifically from 10 to 300 Å similarly to the well layer.

[0079] In the embodiments to be described later, barrier layers dopedwith p-type impurity are used. The p-type impurity concentration is in arange from 5×10¹⁶/cm³ to 1×10²⁰/cm³, and preferably in a range from5×10¹⁶/cm³ to 1×10¹⁸/cm³. This is because p-type impurity concentrationhigher than 1×10²⁰/cm³ does not cause substantial difference in thecarrier concentration while causing deterioration of the crystallinitydue to inclusion of the impurity and an increase in the loss due tolight scattered by the impurity, thus resulting in lower efficiency oflight emission in the active layer. When impurity concentration iswithin ×10¹⁸/cm³, the lowering of efficiency of light emission due toincreasing impurity can be suppressed and high carrier concentrationfrom the p-type layer into the active layer can be kept stable. Inaddition, a trace of p-type impurity is preferably included as the lowerlimit of the p-type impurity concentration. This is because a low p-typeimpurity concentration causes the p-type impurity to function as acarrier with a higher probability than in the case of highconcentration. At this time, the barrier layer which includes the p-typeimpurity preferably includes substantially no n-type impurity, in theembodiments described later. This is because the barrier layer whichincludes the p-type impurity having substantially no n-type impurityincluded therein functions as a barrier layer that accelerates theinjection of the carrier from the p-type layer, while the presence ofthe p-type impurity enhances the action. FIG. 14 and FIG. 15 show thelifetime of the device and the reverse withstanding voltage,respectively, as the functions of the n-type impurity concentration inthe barrier layer located nearest to the p-type layer. As will beapparent from these graphs, lifetime of the device and the reversewithstanding voltage decrease sharply as the n-type impurityconcentration increases, resulting in deterioration in the devicecharacteristics. Thus in the nitride semiconductor device of the presentinvention, the barrier layers (the first barrier layer, the barrierlayer B₁ and the first p-type barrier layer) which are nearest to thep-type layer are preferably grown without doping with the n-typeimpurity or not doping substantially with the n-type impurity, morepreferably include p-type impurity, and most preferably include p-typeimpurity without including n-type impurity. This is because the absenceof n-type impurity makes the injection of the carrier from the p-typelayer efficient, while the presence of the p-type impurity acceleratesthe injection of the carrier, and the combination of these conditions,namely including p-type impurity without including n-type impurity,makes it possible to inject a large amount of carrier efficiently fromthe p-type layer.

[0080] (Doping with n-type Impurity)

[0081] According to the present invention, the active layer compriseswell layers which include n-type impurity 5×10¹⁶/cm³ in concentration orhigher and barrier layers, and preferably one or more of the well layersand/or barrier layers in the active layer is undoped or does notsubstantially include n-type impurity. This results in the active layerwhich as whole contains n-type impurity while the well layers and/orbarrier layers which constitute a part of the active layer are dopedwith the r-type impurity, thus achieving an efficient distribution ofcarrier concentration for the active layer.

[0082] For the purpose of the present invention, the term undoped meansa nitride semiconductor grown without intentionally doping with n-typeor p-type impurity. At this time, impurity concentration becomes lessthan 5×10¹⁶/cm³. In the present invention, not substantially includingn-type impurity or p-type impurity means that the impurity concentrationis less than 5×10¹⁶/cm³.

[0083] Described above are the explanations of the active layer, thebarrier layers and the well layers not detailed in the preferredembodiments that follow, and will he complemented by the description ofthe embodiments.

[0084] Embodiment 1

[0085] The first embodiment of the nitride semiconductor deviceaccording to the present invention comprises, as shown in FIG. 2 andFIG. 3, an active layer 12 sandwiched by a p-type nitride semiconductorlayer 13 and an n-type nitride semiconductor layer 11, with the activelayer including therein a first barrier layer located at a positionnearest to the p-type nitride semiconductor layer and a second barrierlayer which includes n-type impurity. The first barrier layer is undopedwith the n-type impurity, or has been grown without doping so thatsubstantially no n-type impurity is included therein. For the firstbarrier layer, either the layer nearest to the p-type nitridesemiconductor layer in the active layer (hereinafter referred to as thelayer nearest to the p side) may be a well layer 1 b as shown in FIG. 2,or this layer may be first barrier layer as shown in FIG. 3. Preferably,as shown in FIG. 3, when the layer nearest to the p side in the activelayer is used the first barrier layer, the first barrier layer can beprovided in the active layer in contact with the p-type nitridesemiconductor layer, so that the p-type nitride semiconductor layer 13,the first barrier layer 2 d in the active layer 12 and the continuousp-type layer can be formed into the active layer as shown in FIG. 3.This makes it possible to efficiently inject the carrier from the p-typelayer into the active layer, thereby decreasing the loss in driving thedevice and improving the device characteristics, particularly thereverse withstanding voltage and the device lifetime. In the case shownin FIG. 3, in contrast, since the well layer 1 b is interposed betweenthe p-type nitride semiconductor layer 13 and the first barrier layer 2c, continuous p-type layer may not be formed. However, since the firstbarrier layer 2 c is the layer nearest to the p side in the activelayer, an effect not conspicuous as but similar to that of the formercase (the case of FIG. 3) is obtained thus making efficient injection ofthe carrier possible, although the effect is somewhat lower than that ofthe former case (the case of FIG. 3). At this time, the well layerpreferably undoped as described above and, when the n-type impurity isincluded, the concentration thereof is preferably lower than that in thebarrier layer.

[0086] Or the other hand, in case the barrier layer which contains thep-type impurity (hereinafter referred to as p-type barrier layer) is thebarrier layer not located at the position nearest to the p side in theactive layer, making the barrier layer 2 c a p-type barrier layer inFIG. 3, for example, results in deteriorated device characteristics.This is because the diffusion path length of the p-type carrier (hole)is significantly shorter than that of the n-type so that no substantialcontribution is achieved to the improvement in the efficiency of carrierinjection into the active layer, while injection of the n-type carrieris impeded leading to greater loss. This problem is most conspicuouswhen the barrier layer not located at the position nearest to the p sideincludes the p-type impurity.

[0087] The second barrier layer may be provided on the n-type nitridesemiconductor layer side (hereinafter referred to as n side) adjacent tothe first barrier layer, but is preferably provided via at least onewell layer as shown in FIG. 2, FIG. 3. In this constitution, since theactive layer has such a structure that comprises the first barrier layerwhich includes the p-type impurity is located nearest to the p side viaone or more well layer, and the second barrier layer which includes then-type impurity, injection of carrier into one or more well layer beingsandwiched can be made more efficiently than in the case of adjacentarrangement without interposing the well layer. Therefore, it ispreferable that the barrier layer nearest to the n side, namely thebarrier layer 2 a in FIGS. 2, 3, is at least the second barrier layer,that is, the first barrier layer and the second barrier layer are theoutermost barrier layers of the active layer and located on the p sideand on the n side, respectively. Moreover, the second barrier layer maybe provided by single, or may be all the barrier layers except for thefirst barrier layer. Accordingly, in the first embodiment, the barrierlayer nearest to the p side is the first barrier layer and the barrierlayer nearest to the n side is the second barrier layer and morepreferably, in addition to the constitution described above, all thebarrier layers except for the barrier layer nearest to the p side arethe second barrier layer. This constitution makes it possible to injecta large amount of carriers efficiently under high output operation ofthe device, resulting in improved reliability of the device under highoutput power operation. At this time, in addition to the constitution ofthe barrier layer nearest to the p side being the first barrier layerand the barrier layer nearest to the n side being the second barrierlayer, such a constitution may also be possible as the barrier layersecond nearest to the p side or the second nearest to the p side and thesubsequent barrier layers are used as the p-type barrier layers. In theconstitution of the first barrier layer 2 d, the p-type barrier layer 2c and the second barrier layer 2 a in FIG. 3, difference in thediffusion path length of the carrier impedes the improvement in theefficiency of carrier injection and recombination, leading to greaterloss.

[0088] The first barrier layer of the first embodiment of the presentinvention will be described in more detail below. It is an importantfactor in achieving the effect described above that the first barrierlayer does not substantially include the n-type impurity as well as toinclude the p-type impurity. That is, similar effect can be achieved bynot including the n-type impurity as that achieved by including thep-type impurity. This is because, since the first barrier layer does notinclude the n-type impurity, it is made possible to inject a largeamount of the carrier efficiently from the p-type layer into the activelayer or into the well layer nearest to the p side in the vicinity ofthe interface between the p-type layer or in the vicinity of the firstbarrier layer in the active layer, thereby improving the devicecharacteristics similarly to the case described above. Conversely,making the first barrier layer virtually void of the n-type impurityleads to the improvement in the device characteristics. Preferably,having the p-type impurity included without substantially including then-type impurity makes the effect described above remarkable. The secondbarrier layer is preferably grown without doping with the p-typeimpurity, or grown so as not to substantially include p-type impurity.

[0089] In the first embodiment of the present invention, lifetime of thedevice can be increased by making the thickness of the first barrierlayer larger than the thickness of the second barrier layer. This isbecause as a sufficient space is secured as the first barrier layerwhere much p-type impurities exist during high output operation inaddition to the relation with the first p-type nitride semiconductorlayer to be described later, stable injection of carriers andrecombination are ensured even with high output power. Conversely, thefact that the second barrier layer is thinner than the first barrierlayer means that the distance of the well layer in the active layer fromthe n-type laser side is smaller, thereby accelerating the carrierinjection from the n-type layer side to each well layer. At this time,by providing the second barrier layer by single, or setting all thebarrier layers except for the first barrier layer as the second barrierlayer, the distances of all the well layer from the n-type layer sidecan be made smaller, thereby making the carrier injection from then-type layer side efficient.

[0090] Embodiment 2

[0091] In the second embodiment of the present invention, the activelayer has L (L≧2) barrier layers and, with a barrier layer located at aposition nearest to the n-type nitride semiconductor layer denoted as B₁and a barrier layer which is the ith (i=1, 2, 3, . . . , L) layer fromsaid barrier layer B₁ to said p-type nitride semiconductor layer sidedenoted as barrier layer B_(i), the barrier layers B_(i) of i=1 to i=n(1<n<L) have n-type impurity, while barrier layer B_(L) of i=L does notsubstantially include n-type impurity. The barrier layer B correspondsto the first barrier layer of the first embodiment and is the barrierlayer located nearest to the p side, while the action of the barrierlayer B_(L) is similar to that of the first embodiment. Therefore, thebarrier layer B_(L) in the second embodiment contains at least thep-type impurity and preferably does not substantially include the n-typeimpurity, so that the p-type carrier is injected selectively into thebarrier layer B_(L), thereby making the efficient injection of thecarrier possible. Also because the harrier layers B_(i) of i=1 to i=ninclude the n-type impurity so that n barrier layers are doped with then-type impurity starting with the barrier layer nearest to the n-typelayer thereby increasing the carrier concentration, the carrier isinjected smoothly from the n-type layer into the active layer, resultingin accelerated injection and recombination of the carrier and improveddevice characteristics. At this time, the well layer may be eitherundoped or doped with the n-type impurity. Particularly when the deviceis driven with a large current (high output LD or LED), making the welllayer undoped or not substantially including the n-type impurityaccelerates the recombination of the carrier in the well layer so thatthe nitride semiconductor device of high device characteristics and highreliability can be obtained.

[0092] The number n in the second embodiment is required to satisfy atleast the condition of 0<n<L, and preferably satisfy the condition ofn_(m)<n<L and n_(m)=L/2 (n_(m) is an integer with the fraction roundedoff). This is because it is made possible to inject the carrier from then-type layer deep into the active layer (to the p-type layer side) byhaving the n-type impurity included in half or more of the barrierlayers in the active layer. This acts advantageously particularly in thecase of a multi-quantum well structure wherein the number of the welllayers in the active layer is 3 or more, or the number of layers stackedin the active layer is 7 or more. Specifically, in FIGS. 2, 3, thebarrier layers 2 a, 3 a are doped with the n-type impurity as thebarrier layers B_(i), the barrier layers 2 c (FIG. 2) or the barrierlayers 2 d (FIG. 3) is doped with the p-type impurity as the barrierlayers B_(L), and other barrier layers sandwiched by the barrier layersB₁ and the barrier layers B_(L) are undoped, thereby to constitute theactive layer.

[0093] In the second embodiment, by making the barrier layer B_(L)thicker than the barrier layer B_(i) (i≠L) as described above, in a highoutput device which requires stable injection of carriers in a largeamount into the well layer, since the barrier layer B_(L) located at aposition nearest to the p-type layer (near the entrance of injecting thecarrier from the p-type layer) has a large space where the p-typecarriers exist, the carriers of high concentration can be injected in astable manner, so that reliability of the device such as lifetime isimproved.

[0094] Embodiment 3

[0095] The active layer 107 has MQW structure wherein In_(x1)Ga_(1-x1)Nwell layer (0<x₁<1) and In_(x2)Ga_(1-x2)N barrier layer (0<x₂<1, x₁>x₂)are stacked alternately a proper number of times in the order of barrierlayer, well layer, barrier layer, with both ends of the active layerbeing the barrier layers. The well layers are grown undoped. On theother hand, all barrier layers except for the last barrier layer whichadjoins the p-type electron confinement layer 108 are doped with ann-type impurity such as Si or Sn, while the first barrier layer is grownundoped. The last barrier layer includes a p-type impurity such as Mgwhich has diffused therein from the adjacent p-type nitridesemiconductor layer.

[0096] As the barrier layers except for the first barrier layer aredoped with the n-type impurity, the initial electron density in theactive layer becomes higher and the efficiency of injecting electronsinto the well layer is increased, thus resulting in improved efficiencyof laser emission. The last barrier layer, on the other hand, is locatednearest to the p-type layer and therefore does not contribute to theinjection of electrons into the well layer. Therefore, the efficiency ofinjecting holes into the well layer can be improved by virtually dopingthrough the diffusion of the p-type impurity from the p-type layer,without doping the first barrier layer with the n-type impurity. Alsobecause the first barrier layer is not doped with the n-type impurity,such a problem can be eliminated that the carrier mobility decreases dueto the coexistence of impurities of different types in the barrierlayer.

[0097] An example of the active layer is described below. The activelayer 107 is formed on the n-type nitride semiconductor layers 103 to106. As described previously, the active layer 107 has the MQW structurewherein In_(x1)Ga_(1-x2)N well layer (0<x₁<1) and In_(x2)Ga_(1-x2)Nbarrier layer (0≦x₂<1, x₁>x₂) are stacked alternately a proper number oftimes, with both ends of the active layer being the barrier layers. Thewell layer is formed undoped. All barrier layers except for the firstbarrier layer are doped with an n-type impurity such as Si or Snpreferably in a concentration from 1×10¹⁷/cm³ to 1×10¹⁹/cn³.

[0098] The last barrier layer is grown undoped, and includes a p-typeimpurity such as Mg in a concentration from 1×10¹⁶/cm³ to 1×10¹⁹/cm³through the diffusion from the p-type electron confinement layer 10B tobe grown next. When growing the first barrier layer, it may also begrown while doping with p-type impurity such as Mg in a concentrationnot higher than 1×10¹⁹/cm³. The east barrier layer is formed to bethicker than the other barrier layers in order to suppress the effect ofdecomposition by gas etching when growing the p-type electronconfinement layer 108. Thickness of the first barrier layer, ispreferably 1.2 to 10 times the thickness of the other barrier layers,more preferably 1.1 to 5 times, although it depends on the conditions ofgrowing, the p-type electron confinement layer 108. With thisconstitution, the first barrier layer serves as the protective filmwhich prevents the active layer that includes In from decomposing.

[0099] Embodiment 4

[0100] The fourth embodiment of the present invention has such aconstitution as a first p side barrier layer disposed at a positionnearest to the p-type nitride semiconductor layer and a second n sidebarrier layer which is disposed at a position near to the n-type nitridesemiconductor layer are provided as the outermost layers in the activelayer, and the first p side barrier layer includes p-type impurity whilethe second n side barrier layer includes n-type impurity. In thisconstitution, as shown in FIG. 3, the active layer is sandwiched by thefirst p side barrier layer 2 a and the second n side barrier layer 2 dand has the well layer 1 and the barrier layers 2 b, 2 c. Since thefirst p side barrier layer is provided as the layer disposed at aposition nearest to the p side in the active layer, the carriers can beinjected efficiently from the p-type layer, while injection of thecarriers from the n-type layer can be made satisfactorily by providingthe second n side barrier layer as the layer disposed at a position nearto the n-type layer in the active layer. As a result, efficientinjection of the carriers from the n side layer and the p side layerinto the active layer and recombination thereof are made possible, thusachieving improvements in the reliability and lifetime of the device ofhigh output power. At this time, the p-type layer and the n-type layerare preferably provided to adjoin the first p-type barrier layer 2 d andthe second n-type barrier layer 2 a as shown in FIG. 3, which causes thep-type layer and the n-type layer to be connected directly to the activelayer, thereby achieving better injection of the carriers. At this time,while the barrier layers sandwiched by the first p side barrier layer 2d and the second n side barrier layer, the barrier layers 2 b, 2 c inthe case of FIG. 3, for example, are not limited, it is preferable thatthe layers are doped with the n-type impurity which enables efficientinjection of the carriers from the n-type layer thereby improving thereliability of the device.

[0101] By making the first p side barrier layer and said second n sidebarrier substantially equal in thickness, barrier layers are providedwith the outermost layers of the active layer being substantiallysymmetrical, so that variations in the devices are suppressed thereby toimprove the yield of production. This is believed to be the result of,through detailed mechanism is not known, the first p side barrier layerand the second n side barrier layer which function as the entrance forinjecting the carriers of the p-type layer and the n-type layer beingconfigured symmetrically, which improves the symmetry of the layerstructure of the active layer and results in lower threshold current andstable lifetime of the device.

[0102] In the fourth embodiment, the active layer has two or more welllayers and a third barrier layer disposed between the well layers, whilethickness of the third barrier layer is smaller than the thickness ofthe first p side barrier layer and the second n side barrier layer. Thisconstitution makes it possible to improve the device characteristicsfurther. That is, the second n side barrier layer and the first p sidebarrier layer which are disposed at the outermost positions of theactive layer function as the entrance for injecting the carriers of thep-type layer and the n-type layer, respectively, and have largerthickness than the other barrier layers, so as to secure a space largerenough to hold a large amount of carriers, thus enabling stableoperation of the device even with large current. On the other hand,since the third barrier layer is sandwiched by the well layers, itsuffices to provide the layer so that the carriers are injected intoeach well layer and communication between the well layers is achieved,thus it is not necessary to form in large thickness unlike the outermostbarrier layers. In addition, with such a structure as the thick barrierlayers disposed as the outermost layers and thin barrier layers disposedin the middle of the active layer, the first p side barrier layer andthe second n side barrier layer which are thicker than the third barrierlayer function as strong barrier layers located on the opposite sideswith respect to the n-type layer and the p-type layer while the carriersfrom the n-type layer and the p-type layer are injected by means of theouter barrier layers, thereby accelerating the injection of the carrierinto each well layer and the light emission recombination. Making thethird barrier layer thinner than the outer barrier layers makes itpossible to keep the total thickness of the active layer small, thuscontributing to the decrease in the threshold current density and thevalue of Vf.

[0103] As will be understood from the foregoing description, the firstto fourth embodiments have the following features.

[0104] In the first to fourth embodiments, injection of the carriersinto the active layer is accelerated because the barrier layers (thefirst barrier layer, the barrier layer B₁ and the first p-type barrierlayer) which are located nearest to the p-type layer in the active layerdo not substantially-contain the n-type impurity, thereby achieving thenitride semiconductor device having excellent device lifetime and highoutput power. Moreover, by including the p-type impurity, injection ofcarrier and light emission recombination can be done efficiently evenwith a large amount of carriers, thereby achieving the nitridesemiconductor device having excellent device lifetime and high outputpower. At this time, when the barrier layer located nearest to thep-type layer includes the p-type impurity, the layer is preferably grownundoped or so as not to substantially include r-type impurity. This isbecause, in case the n-type purity is included when the barrier layerlocated nearest to the p-type layer includes the p-type impurity,carrier injection from the p-type layer rends to be impeded leading toweaker effect of injecting a large amount of carriers efficiently, thusresulting in shorter device lifetime and lower output power.

[0105] Embodiment 5: Laser Device

[0106] The fifth embodiment of the nitride semiconductor device of thepresent invention is a laser device having at least such a structure asan active layer is sandwiched by an n-type cladding layer and a p-typecladding layer in a p-type nitride semiconductor layer and an n-typenitride semiconductor layer. An optical guide layer which interposes theactive layer may also be provided between the cladding layer and theactive layer. In the forth embodiment of the present invention,waveguide structure may be applicable not only laser device but alsoedge emitting LED or super luminescent diode.

[0107]FIG. 1 is a sectional view showing an example of the nitridesemiconductor layer of the present invention. An active layer 107 madeof In_(x)Ga_(1-x)N (0≦x<1) is sandwiched by n-type Al_(y)Ga_(1-y)N(0≦y<1) layers 103 to 106 (every layer has different value of y) andp-type Al_(z)Ga_(1-z)N (0≦z<1) layers 108 to 111 (every layer hasdifferent value of z) on a GaN substrate 101, thus constituting theso-called double hetero structure.

[0108] The n-type cladding layer and the p-type cladding layer are madeof a nitride semiconductor which includes Al, and specificallyAl_(b)Ga_(1-b)N (0<b<1) is preferably used.

[0109] According to the present invention, there is no limitation to thecomposition of the optical guide layer which is required only to have asufficient energy band gap for forming the waveguide, whether made insingle film or multiple-film constitution. For example, refractive indexof the waveguide can be made higher by using GaN for wavelengths from370 to 470 and using multiple-film constitution of InGaN/GaN for longerwavelengths. Thus various nitride semiconductors such as InGaN, GaN andAlGaN can be used. The guide layer and the cladding layer may also bemade in super lattice multiple-film structure.

[0110] Now detailed structure of the nitride semiconductor laser shownin FIG. 1 will be describe below. Formed on the substrate 101 via thebuffer layer 102 are the n-type contact layer 103 which is an n-typenitride semiconductor layer, the crack preventing layer 104, the n-typecladding layer 105 and the n-type optical guide layer 106. Layers otherthan the n-type cladding layer 105 may be omitted depending on thedevice. The n-type nitride semiconductor layer is required to have aband gap wide than the active layer at least in a portion which makescontact with the active layer, and therefore preferably has acomposition which includes Al. The layers may also be made n-type eitherby growing while doping with an n-type impurity or by growing undoped.

[0111] The active layer 107 is formed on the n-type nitridesemiconductor layers 103 to 106. The construction of the active layer isas described previously.

[0112] Formed on the last barrier layer are the p-type electronconfinement layer 108 as the p-type nitride semiconductor layer, ap-type optical guide layer 109, a p-type cladding layer 110 and a p-typecontact layer 111. Layers other than the p-type cladding layer 110 maybe omitted depending on the device. The p-type nitride semiconductorlayer is required to have a band gap wide than the active layer at leastin a portion which makes contact with the active layer, and thereforepreferably has a composition which includes Al. The layers may be madep-type either by growing while doping with a p-type impurity or by usingthe diffusion of the p-type impurity from the other layer which adjoinsthereto.

[0113] The p-type electron confinement layer 108 made of a p-typenitride semiconductor layer which includes Al in a proportion higherthan in the p-type cladding layer 110, preferably having a compositionof Al_(x)Ga_(1-x)N (0.1<x<0.5). This layer is also heavily doped with ap-type impurity such as Mg preferably in a concentration from 5×10¹⁷/cm³to 1×10¹⁹/cm³. This makes the p-type electron confinement layer 108capable of effectively confining electrons within the active layer, thusdecreasing the threshold of the laser. The p-type electron confinementlayer 108 may be formed in a thin film of about 30 to 200 Å. Such a thinfilm can be grown at a temperature lower than the temperature forgrowing the p-type optical guide layer 109 and the p-type cladding layer110. By forming the p-type electron confinement layer 108 as describedabove, decomposition of the active layer which includes In can besuppressed compared to the case where the p-type optical guide layer 109and the like are formed directly on the active layer.

[0114] The p-type electron confinement layer 109 plays the role ofsupplying the p-type impurity through diffusion into the last barrierlayer which is grown undoped. These layers work in cooperation toprotect the active layer 107 from decomposition and improve theefficiency of injecting holes into the active layer 107. That is, byforming the undoped In_(x2)Ga_(1-x2)N well layer (0≦x₂<1) as the lastlayer of the MQW active layer to be thicker than the other barrierlayers and growing thereon the thin film made of p-type Al_(x)Ga_(1-x)N(0.1<x<0.5) heavily doped with the p-type impurity such as Mg at a Lowtemperature, it is made possible to protect the active layer 107 fromdecomposition and improve the efficiency of injecting holes into theactive layer through diffusion of the p-type impurity such as Mg fromthe p-type Al_(x)Ga_(1-x)N layer into the undoped In_(x2)Ga_(1-x2)Nlayer.

[0115] Among the p-type nitride semiconductor layers, ridge stripe isformed up to midway in the p-type optical guide layer, and thesemiconductor laser is made by forming protective films 161, 162, ap-type electrode 120, an n-type electrode 121, a p-type pad electrode122 and an n-type pad electrode.

[0116] (Eelectron Confinement Layer: First p-type Nitride SemiconductorLayer)

[0117] According to the present invention, the first p-type nitridesemiconductor layer is preferably provided as the p-type nitridesemiconductor layer particularly in the laser device. The first p-typenitride semiconductor layer is made of a nitride semiconductor whichincludes Al, and specifically Al_(a)Ga_(1-a)N (0<a<1) is preferablyused. The proportion γ of Al is determined so the layer functions as theelectron confinement layer when used in a laser device which requires itthat a sufficiently larger band gap energy is provided (offset) thanthat of the active layer, and is set at least in a range of 0.1≦γ<1, andpreferably in a range of 0.2≦γ<0.5. This is because the value of y lessthan 0.1 makes it unable to fully function as the electron confinementlayer in the laser device. When the value of γ is 0.2 or greater,electrons (carrier) can be sufficiently confined and carrier overflowcan be suppressed. When the value of γ is not larger than 0.5, the layercan be grown while minimizing the occurrence of cracks. Further, thevalue of γ is preferably set to 0.35 or less which allows it to achievegood crystallinity. At this time, proportion of Al in the composition isset higher than in the p-type cladding layer, because confinement of thecarrier requires a nitride semiconductor having higher proportion of Althan in the cladding layer which confines light. The first p-typenitride semiconductor layer can be used in the nitride semiconductordevice of the present invention and, particularly in the case of a laserdevice which is driven with a large current while injecting a largeamount of carriers into the active layer, the carriers can be confinedmore efficiently than in the case where the first p-type nitridesemiconductor layer is not provided, making applicable to high outputLEDs as well as the laser device.

[0118] Thickness of the first p-type nitride semiconductor layer of thepresent invention should be not larger than 1000 Å, preferably 400 Å orsmaller. This is because the nitride semiconductor which includes Al hasbulk resistance higher than other types of nitride semiconductor(without Al content), and therefore makes a layer of extremely highresistance when formed with a thickness greater than 1000 Å, thusresulting in a significant increase in the forward voltage Vf. When thethickness is not larger than 400 Å, the increase in Vf can be kept at alow level. More preferably the thickness is 200 Å or less which makes itpossible to suppress the increase in Vf even lower. Lower limit of thethickness of the first p-type nitride semiconductor layer is 10 Å orlarger, preferably 50 Å or larger, which allows the electron confinementlayer to function satisfactorily.

[0119] In the laser device, the first p-type nitride semiconductor layeris provided between the active layer and the cladding layer so as tofunction as an electron confinement layer or, when a guide layer isadditionally provided, provided between the guide layer and the activelayer. At this time, the distance between the active layer and the firstp-type nitride semiconductor layer is not larger than 1000 Å whichenables it to function as the carrier confinement layer, and preferably500 Å or smaller for better carrier confinement. This is because thefirst p-type nitride semiconductor layer has better effect of carrierconfinement when nearer to the active layer. Moreover, since it is notnecessary to have another layer between the active layer and the firstp-type nitride semiconductor layer in most cases in the laser device, itis usually most preferable to provide the first p-type nitridesemiconductor layer so as to make contact with the active layer. At thistime, in case the crystallinity is adversely affected by providing thelayer located nearest to the p-type nitride semiconductor layer in theactive layer of the quantum well structure and the first p-type nitridesemiconductor layer in contact with each other, a buffer layer may beprovided between both layers when growing the crystal, in order to avoidthe adverse effect. For example, a buffer layer made of GaN maybeprovided between The outermost p side layer of the active layer and thefirst p-type nitride semiconductor layer such as InGaN or AlGaN, or abuffer layer made of nitride semiconductor which includes Al in lowerproportion than that in the first p-type nitride semiconductor layer maybe provided.

[0120] The positional relationship between the first p-type nitridesemiconductor layer and the active layer, particularly the distance fromthe well layer is an important factor that determines the thresholdcurrent density and lifetime of the laser device. Specifically, thethreshold current density can be made lower as the first p-type nitridesemiconductor layer is located nearer to the active layer, but this alsoleads to shorter lifetime of the device. This is supposedly because, asdescribed above, the first p-type nitride semiconductor layer has aresistance far higher than that of the other layer and generates greaterheat during operation of the device thus reaching a higher temperaturein the device. This may be affecting the active layer and the welllayers which are vulnerable to heat, thus causing the device lifetimesignificantly. On the other hand, as described above, the first p-typenitride semiconductor layer which bears the function of carrierconfinement can perform more effective carrier confinement function whenlocated nearer to the active layer, with the effect diminishing as thedistance from the active layer increases. Therefore, keep the lifetimeof the device from decreasing, distance dB between the well layer 1 andthe first p-type nitride semiconductor layer 28 should be 100 Å orlarger, preferably 120 Å or larger, and most preferably 140 Å or largeras shown in FIG. 8A, When the distance dB between the well layer and thefirst p-type nitride semiconductor layer is shorter than 100 Å, thedevice lifetime decreases sharply. When the distance is not less than120 Å, the device lifetime can be increased significantly. When thedistance is larger than 150 Å, the device lifetime increases further,while the threshold current density begins to gradually increase. Whenthe distance is larger than 200 Å, she threshold current density showsclear tendency to increase and, beyond 400 Å, the threshold currentdensity increases sharply. Accordingly, upper limit of the distance isset within 400 Å, preferably within 200 Å. The mechanism behind this isbelieved to be such that the efficiency of carrier confinement decreasesas the first p-type nitride semiconductor layer departs from the welllayer, which becomes the major cause and increase in the thresholdcurrent density, resulting in the decrease in the efficiency of lightemission.

[0121] The well layer used as the datum for distance is the well layer 1b which is disposed on the side of the n-type layer while adjoining thebarrier layer 2 c provided nearest to the p-type layer 13 in the activelayer. In case the first p-type nitride semiconductor layer is providedin contact with the active layer of quantum well structure, either theconstitution shown in FIG. 8A wherein the first p-type nitridesemiconductor layer 28 is provided in contact with the barrier layer 2 clocated nearest to the p-type layer side or the constitution shown inFIG. 8B wherein the well layer 4 is provided between the first p-typenitride semiconductor layer 28 and the barrier layer 2 c located nearestto the p-type layer 13 side may be employed. In the constitution wherethe well layer 4 is provided between the first p-type nitridesemiconductor layer 28 and the barrier layer 2 c located nearest to thep-type layer 13 side, the well layer 4 becomes too near to the p-typelayer 13, so that most of the carriers injected from the p-type layerpass through the well layer 4 without experiencing light emissionrecombination in the well layer 4, thus the function of the well layeris not performed. In case the barrier layer 2 c located nearest to thep-type layer side includes the p-type impurity, the carriers can beinjected better into the well layers 1 a, 1 b which are located nearerto the n-type layer side than the barrier layer 2 c, while the carrierspass through the well layer 4 which is located nearer to the p-typelayer side than the barrier layer 2 c, thus further losing thecontribution to light emission recombination and the function of thewell layer is rapidly lost. For this reason, between the well layer 4and the first p-type nitride semiconductor layer shown in FIG. 8B, thereis no change in the characteristic due to the distance as describedabove, and the distance d_(B) from the well layer becomes the distancefrom the well layer located at a position nearer to the n-type layerside than the barrier layer nearest to the p-type layer regardless ofthe well layer 1 c located at a position nearer to the p-type layer sidethan the barrier layer nearest to the p-type layer. And even when a welllayer which becomes the layer nearest to the p-type layer is provided inthe active layer, change in the characteristics due to the distanced_(B) as described above is observed. Such a well layer which is locatedat a position nearer to the p-type layer side than the barrier layernearest to the p-type layer does not fully function as a well layer, andalso cause the device characteristics such as lifetime to deterioratecompared to a case where the well layer is not provided. Therefore, sucha constitution is preferably employed as a layer located nearest to thep-type layer in the active layer is used as the barrier layer withoutproviding the well layer, as shown in FIG. 8A rather than that shown inFIG. 8B.

[0122] In case the first p-type nitride semiconductor layer 28 isprovided in contact with the barrier layer 2 c located nearest to thep-type layer side, the barrier layer 2 c (barrier layer nearest to thep-type layer side) may be provided between the well layer and the firstp-type nitride semiconductor layer, so as to determine the distanced_(B) by the thickness of this barrier layer. Therefore, thickness ofthe barrier layers (the first barrier layer, the barrier layer B_(L) andthe first p-type barrier layer) which are located nearest to the p-typelayer becomes an important factor that determines the characteristics ofthe nitride semiconductor device. In addition, since the increase in thethreshold current density of a laser device is caused mainly by theconfinement of the carrier described above, the relationship between theactive layer and the first p-type nitride semiconductor layer applieshere again.

[0123] The first p-type nitride semiconductor layer of the presentinvention is usually doped with a p-type impurity, and is doped with ahigh concentration in order to improve the carrier mobility in the caseof a laser device or a high power LED which are driven with a largecurrent. Specifically, doping concentration is 5×10 ¹⁶/cm³ or higher,and preferably 1×10¹⁸/cm³ or higher. In the case of a device driven witha large current, doping concentration is 1×10¹⁸/cm³ or higher, andpreferably 1×10¹⁹/cm³ or higher. Concentration of the p-type impurity isnot limited, but is preferably 1×10²¹/cm³ or higher. However, when theconcentration of the p-type impurity is too high, bulk resistance tendsto increase resulting in higher value of Vf. In order to avoid thisproblem, minimum necessary concentration of p-type impurity to securethe required level of carrier mobility is preferably provided. In case ap-type impurity which has a strong tendency to diffuse such as Mg isused, the first p-type nitride semiconductor layer may be grown withoutdoping and then doped by the diffusion of impurity from an adjacentlayer,. for example, an optical guide layer. Or, alternatively, in casethe first p-type nitride semiconductor layer is grown without dopingwhile a layer doped with the p-type impurity exists other than theadjacent layer or the p-type impurity diffused region, and diffusion ofimpurity does not occur in the first p-type nitride semiconductor layer,then the layer may be provided undoped when the thickness allows thecarrier to tunnel therethrough.

[0124] In addition to the above, in the laser device of the presentinvention, in case a p-type optical guide layer is provided in contactwith the first p-type nitride semiconductor layer, a good optical guidelayer is obtained by doping with a p-type impurity by the diffusion fromthe first p-type nitride semiconductor layer. Since the p-type impurityin the guide layer scatters light in the optical waveguide, inclusion ofimpurity with as low concentration as possible within such a range thatensures electrical conductivity is preferable for improving the devicecharacteristics. However, doping with p-type impurity when growing thep-type optical guide layer has a problem of difficulty to control thedoping with impurity in a low concentration region where the loss oflight can be kept low. This is because, while the nitride semiconductordevice generally has such a structure as an n-type layer, an activelayer and a p-type layer are stacked in this order, it is necessary toprevent In from decomposing during the subsequent process of growing thelayers because of InGaN included in the active layer, when thisstructure is grown. Although a method of growing the p-type layer at alow temperature in a range from 700 to 900° C. is commonly employed, itbecomes difficult to control the doping impurity concentration becauseof the low temperature. Also Mg is commonly used as the p-type impurity,it is relatively difficult to control the doping impurity concentration,and variations may be caused in the device characteristics when dopingthe impurity in the low concentration region when growing.

[0125] Therefore, it is preferable that the first p-type nitridesemiconductor layer plays the role of the impurity supplying layer bydoping the first p-type nitride semiconductor layer with the impurity ina high concentration during growth thereof, while giving considerationto the diffusion of the p-type impurity into the optical guide layer.Further in the embodiments described above, the barrier layers (thefirst barrier layer, the barrier layer B and the first p-type barrierlayer) which are disposed in contact with the first p-type nitridesemiconductor layer can also be caused to play the role of the impuritysupplying layer by doping with the p-type impurity, too.

[0126] In the laser device of the present invention, as shown in theembodiments, an insulation film which becomes a buried layer is formedin the side face of a ridge after forming the ridge. For the buriedlayer, a material other than SiO₂, preferably an oxide which includes atleast one kind of element selected from among a group consisting of Ti,V, Zr, Nb, Hf and Ta, or at least one of SiN, BN, SiC and AlN is usedand, among these, it is particularly preferable to use an oxide of Zr orHf, or BN, SiC. As the buried layer, half-insulating i-type nitridesemiconductor; a nitride semiconductor of conductivity type opposite tothat of the ridge (n-type in the case of the embodiment); a nitridesemiconductor including Al such as AlGaN (when current pinching layer isneeded) can be used. Alternatively, in order to form light wave guide,the buried layer is made to absorb much more light than the ridge partby employing nitride semiconductor layer including In (such as InGaN) asthe buried layer. Such a structure may also be employed as ions ofelements such as B or Al are injected without forming a ridge by etchingor the like, while forming a region without the ion being injected whichis the current flowing region. At this time, nitride semiconductorrepresented as In_(x)Al_(1-y)Ga_(1-x-y)N, 0≦x≦1, 0≦y≦1, x+y=1).

[0127] The first p-type nitride semiconductor layer of the presentinvention performs carrier confinement as described above, and the firstp-type nitride semiconductor layer can be applied as a cladding layereven when only the classing layer for carrier confinement is providedwithout needing an optical confinement cladding layer in a lightemitting device, as shown in an embodiment.

[0128] Moreover, while the first p-type nitride semiconductor layer hassuch a constitution as band offset is provided between the active layerin order to confine electrons in the active layer, namely making theband gap energy higher than that in the active layer with a voltagebarrier between both layers, it is preferable to provide band gap energyhigher than that in the guide layer in a laser device of SCH structure.In case cladding layer comprising two or more layers of different bandgap energies is provided, the first p-type nitride semiconductor layeris provided in the cladding layer on the active layer side, preferablywith band gap energy higher than that in the other layers. Specifically,such a structure may be employed as the first p-type nitridesemiconductor layer having a high band gap energy is used as the firstcladding layer and a second cladding layer having lower band gap energyis disposed more distantly from the active layer than the first claddinglayer, such as the structure of the first embodiment minus the guidelayer.

[0129] As described above, when the first barrier layer (barrier layernearest to the p side) is considered based on the first p-type nitridesemiconductor layer, with regards to the carrier confinement in theactive layer being determined by the first p-type nitride semiconductorlayer which plays the role of carrier confinement in the band structureshown in FIG. 8A and 8B, the active layer may be the region from theposit ion of making contact with the first p-type nitride semiconductorlayer in the present invention. That is, in case the barrier layernearest to the p side has a function different from that of the otherbarrier layer, because of the close relationship with the first p-typenitride semiconductor layer which plays the role of carrier confinementas described above, the region from the interface in contact with thefirst p-type nitride semiconductor layer can be regarded as the activelayer. Thus in consideration of the fact that the region from theposition of making contact with the first p-type nitride semiconductorlayer is the active layer, the effect of the first barrier layer, thebarrier layer nearest to the p side of the present invention can beachieved even when some layer, such as the well layer 4 shown in FIG.8B, for example, is interposed between the first barrier layer and thefirst p-type nitride semiconductor layer. Specifically, in addition tothe embodiment shown in FIG. 8B, a layer having an intermediate band gapenergy may be interposed between the first barrier layer and the firstp-type nitride semiconductor layer. Moreover, as described above, sincethe first barrier layer, the barrier layer nearest to the p side have afunction significantly different from the other barrier layer, forexample the barrier layer sandwiched by the well layers, the layer mayhave different composition and band gap energy.

[0130] By setting the ridge width in a range from 1 μm to 3 μm,preferably from 1.5 μm to 2 μm, light source for an optical disk systemhaving good spot shape and beam configuration can be obtained. The laserdevice of the present invention is now limited to the refractive indexguiding type waveguide of ridge structure, and gain guiding type may beemployed. Also the BH structure where ridge side face is buried byregrowth, a structure having a ridge buried by regrowth, or a structurehaving current pinching layer may be employed, and the active layerdescribed above is effective for any laser device structure.

[0131] Embodiment 6

[0132] The sixth embodiment is the nitride semiconductor devicedescribed above having such a laser device structure as the active layerof quantum well structure is sandwiched between an upper cladding layermade of a nitride semiconductor including Al and a lower cladding layermade of a nitride semiconductor including Al, wherein the upper claddinglayer and the lower cladding layer include Al in mean proportion x being0<x≦0.05. This constitution makes it possible to loosen the confinementin the optical waveguide sandwiched by the upper cladding layer and thelower cladding layer by controlling the proportion of Al in the claddinglayers to 0.05 or less, and suppressing the self-excited oscillation bycontrolling the thickness ratio of the barrier layer and the well layerof the active layer, thereby improving the output characteristic and thelifetime of the device. The laser device made in this constitution iscapable of continuous oscillation with an output power of 5 to 100 mW,thus making a laser device having characteristics suitable for thereading and writing light sources of an optical disk system whileachieving a longer lifetime of the device. Preferably, as shown in FIGS.3, 4, 6, 7, an optical guide layer is provided between the upper and thelower cladding layers so as to difference in the refractive index in thecladding layers and spread the distribution of light, which causes thelight to be distributed widely in the guide layer thereby decreasing theloss due to leaking light.

[0133] When the well layer has thickness of 40 Å or larger and thethickness ratio R_(t) is in a range from ⅓ to 1 in the active layer, thedevice characteristics can be improved in the first to fourthembodiments and in the fifth embodiment. Although the mechanism whichimproves the device characteristics is not known, such a structure hasbeen used in the prior art as the probability of light emissionrecombination to occur in the well layer is increased by providing abarrier thickness sufficiently thicker than the well layer. In contrast,in the active layer described above, the well layer is made as thick as40 Å or larger and the barrier layer is made thinner compared to thewell layer. Thus the thick well layer provides greater region for lightemission recombination to occur, and the thin barrier layer providedbetween the well layers allows the carriers to be injected evenly intothe well layers, thus increasing the probability of light emissionrecombination to occur. In the case of a high output device, while alarge amount of carriers are injected into the well layer because ofdriving with a large current, the thick well layer provides greaterregion for light emission recombination to occur, and the thin barrierlayer tends to enable uniform injection into the well layers passingthrough the barrier layer.

[0134] When the well layer has thickness of 40 Å or larger and thethickness ratio R_(t) (R_(t)=[thickness of well layer]/[thickness ofbarrier layer]) is in a range from ⅓ to 1 in the active layer, the laserdevice having excellent characteristics for the light source of anoptical disk system is obtained. This is because making the well layer40 Å or thicker results in the long lifetime of the device as shown inFIG. 12, and setting in the range described above keeps the value of RIN(relative intensity of noise) low. More preferably, thickness of thewell layer is set to 50 Å or larger which makes it possible to increasethe device lifetime further. When the value of R_(t) is 1 or greater,although the value of RIN increases, device lifetime becomes longer anda laser device having a large output power can be made, thus allowingapplications other than the optical disk system, Based on the foregoingdiscussion, thickness of the barrier layer is preferably 40 Å or largerbecause this provides the laser device having long lifetime as shown inFIG. 13.

[0135] Embodiment 7: Number of Well Layers

[0136] In the first to sixth embodiments described above, the number ofwell layers provided in the active layer is set in a range from 1 to 3 anitride semiconductor device having good device characteristicsperformed even under operation with a large current can be obtained. Inthe prior art, while the number of well layers in the active layer hasbeen set in a range from about 4 to 6, the large number of the welllayers increases the probability of the carrier recombination but thetotal thickness of the active layer including the barrier layer becomeslarger and tends to increase the value of Vf. Also it has been foundthat increasing the number of well layers does not cause a correspondingincrease in the probability of the carrier recombination. In the case ofLD which is driven with a large current and high current density, inparticular, this tendency is observed more markedly. In the case of LD,for example, when the number of well layers is changed in multiplequantum well structure, threshold current tends to decrease as thenumber of well layers decreases, and shows sharp decrease while thenumber of well layers decreases from 6 to 4, gradual decrease as thenumber decreases from 4 to 3, reaches a minimum value the number of welllayers is 2 or 3. When the number of well layer is 1, namely in the caseof single quantum well structure, threshold current becomes equal to ora little higher than that of the case where the number of well layers is2 or 3. Similar tendency is seen also in an LED of high output power.

[0137] The accompanying drawings will be described below. FIGS. 2, 3 areschematic sectional views of an embodiment of the present invention,particularly showing a structure of a laser device where an active layer12 is sandwiched by an n-type layer 11 and a p-type layer 13. FIG. 2shows a structure where the active layer 12 is sandwiched by an uppercladding layer 30 and a lower cladding layer 25, a first p-type nitridesemiconductor layer 28 which is an electron confinement layer isprovided between the active layer 12 and the upper cladding layer 30,with the quantum well structure of the active layer 12 being made bystacking pairs of barrier layer 2 a/well layer 1 a repetitively andproviding the barrier layer 2 c at the end. Difference of the structureshown in FIG. 3 from that of FIG. 2 is that upper and lower opticalguide layers 29, 26 are provided between the upper and lower claddinglayers 30, 25 and the active layer 12. FIGS. 4 to 8 and FIG. 10 showstacked structure 20 of or around the active layer 12 and an energy bandgap 21 provided below and corresponding to the stacked structure 20.FIGS. 4, 6 show the quantum well structure of the active layer 12 havingan asymmetrical structure with regard to the film thickness. FIGS. 5, 7show, on the contrary, a symmetrical structure. The number of the welllayers in the active layer is 3 in FIGS. 4, 5 and 2 in FIGS. 6, 7. FIG.5 shows a structure without optical guide layer, while FIGS. 4, 7, 8show a structure having optical guide layer. FIG. 8 shows a structurewhere the active layer 12 and the p-type layer 13 are stacked, showingthe relationship between the first p-type nitride semiconductor layer 28and the active layer in the p-type layer 13, the barrier layer 2 cprovided at a position nearest to the p-type layer side and the welllayer 1 b located nearer to the n-type layer than the barrier layer 2 c.

[0138] Embodiment 8

[0139] The eighth embodiment of the present invention provides a laserdevice having fast response characteristics and RIN suitable for opticaldisk systems such as DVD and CD. Specifically, the active layer ofquantum layer structure comprises a first barrier layer (barrier layerdisposed nearest to the p side) and a second barrier layer the ratioR_(t) of thickness between the well layer and the second barrier layeris in a range of 0.5≦R_(t)≦3. The first barrier layer (barrier layerdisposed nearest to the p side) and the second barrier layer are similarto those of the embodiments described above. With this thickness ratio,it is important that the second barrier layer be the barrier layersandwiched by he well layers in the MQW, namely at the distance betweenthe well layers. As described above, since the barrier layer locatednearest to the p side and the other barrier layers have differentfunctions, for the barrier layers which affect the responsecharacteristics and RIN, the barrier layers other than the first barrierlayer (barrier layer disposed nearest to the p side) are important.Particularly in the MQW, ratio of the thickness of the barrier layersandwiched by the well layers and the thickness of the well layer have agreat influence on the characteristics described above. When thethickness ratio R_(t) is within the range described above, a good laserdevice suited to the light source of an optical disk system is obtained.When the ratio is below 0.5, thickness of the barrier layer becomes toolarge compared to the well layer leading to a degradation in theresponse characteristic. When the ratio exceeds 3, RIN is adverselyaffected thus making a light source having significant noise when highfrequency is superimposed. The ratio is preferably set in a range of0.8≦R_(t)≦2 which leads to a laser device excellent in thecharacteristics described above. At this time, film thickness d_(w) ofthe well layer is preferably in a range of 40 Å≦d_(w)≦100 Å. This isbecause better device lifetime can be achieved as the well layer becomesthicker in the embodiments described above as will be seen from FIG. 12.When the film thickness is larger than 100 Å, degradation of theresponse characteristics and RIN becomes more significant thus makingthe device not suitable for the light source of an optical disk system.The thickness is preferably in a range of 60 Åd_(w)<80 Å. This isbecause, while thicker well layer leads to slower rate of deteriorationwhich is another criterion for evaluating the lifetime of the device,the rate of deterioration shows a sharp decrease when thickness of thewell layer is increased in a range from 40 Å to 80 Å, and shows agradual decrease when the thickness exceeds 80 Å. The film thicknessd_(b) of the second barrier layer is set to 40 Å or larger for theconsideration to the relation between the film thickness and the devicelifetime shown in FIG. 13, and a laser device of excellent devicelifetime is obtained in this range of thickness.

[0140] This embodiment is preferably combined with the first to seventhembodiment. The second barrier layer is at least the barrier layerprovided in the active layer in the embodiment shown in FIGS. 6, 7, andis applied to some of the barrier layers other than the barrier layer 2c provided at a position nearest to the p-type layer side (first barrierlayer). Preferably it is applied to the barrier layer 2 b sandwiched bythe well layers, and most preferably applied to all the barrier layersother than the barrier layer nearest to the p side, because thisimproves the characteristics described above.

EXAMPLE 1

[0141] Now a laser device made of the nitride semiconductor devicehaving the laser device structure as shown in FIG. 8 will be describedbelow as an example.

[0142] While the substrate 101 is preferably made of GaN, a substrate ofa material different from the nitride semiconductor may also be used.The substrate of different material may be made of an insulatingsubstance such as sapphire or spinel (MgAl₂O₄) having principal plane inthe C plane, R plane or A plane, or SiC (6H, 4H, 3C), ZnS, ZnO, GaAs, Sior a material other than nitride semiconductor which has been known tobe capable of growing nitride semiconductor such as an oxide thatundergoes lattice matching with the nitride semiconductor. Preferredmaterial for making the substrate of different material is sapphire orspinel. The substrate of different material may be an off-angle one, inwhich case it has preferably stepwise off-angle construction for thisallows base layer of gallium nitride to grow with good crystallinity.When a substrate of different material is used, devices maybe formed inthe form of single substrate of nitride semiconductor by removing thesubstrate of different material by polishing or other method aftergrowing the nitride semiconductor which makes the base layer on thesubstrate of different material before forming the device structure, oralternatively the substrate of different material may be removed afterforming the devices.

[0143] In case the substrate of different material is used, the nitridesemiconductor can be grown satisfactorily when the devices are formedvia the base layer made of a buffer layer (low-temperature grown layer)and nitride semiconductor (preferably GaN). For the base layer (growthsubstrate) provided on the substrate of different material, nitridesemiconductor grown by ELOG (Epitaxially Laterally Overgrowth) may alsobe used which allows it to obtain a growth substrate of goodcrystallinity. Specific examples of ELOG growth layer include one wherea nitride semiconductor layer is grown on a substrate of differentmaterial, whereon a mask region is formed by, for example, providing aprotective film which makes it difficult to grow the nitridesemiconductor, and a non-mask region is formed in stripes for growingthe nitride scmiconductor, while the nitride semiconductor is grownthrough the non-mask region so that the growth proceeds in the lateraldirection as well as in the direction of thickness, thereby forming alayer with the nitride semiconductor growing also in the mask region. Inother aspect, the layer may also be formed by making an aperture in thenitride semiconductor grown on the substrate of different material andmaking lateral growth from the side face of the aperture.

[0144] (Substrate 101)

[0145] For the substrate, a nitride semiconductor, GaN in this example,is grown into a thick film (100 μm) on a substrate made of a differentmaterial. With the substrate of the different material being removed, anitride semiconductor substrate made of GaN with a thickness of 80 μm isused. Detailed process of forming the substrate is as follows. Asubstrate of different material made of sapphire with the principalplane lying in the C plane having diameter of 2 inches is set in aMOVPEreaction vessel, of which temperature is set to 500° C., and a bufferlayer made of GaN is formed to a thickness of 200 Å by using trimethylgallium (TMG) and ammonia (NH₃). With the temperature raised, a film ofundoped GaN 1.5 μm is grown as a base layer. Then with a plurality ofstriped masks formed on the base layer surface, a nitride semiconductor,GaN in this example, is selectively grown through apertures (windows) ofthe mask. The nitride semiconductor layer formed by a growing processinvolving lateral growth (ELOG) is further grown to become thicker. Thenthe nitride semiconductor substrate is obtained by removing thesubstrate of different material, the buffer layer and the base layer. Atthis time, the mask used in the selective growth is made of SiO₂havingmask width of 15 μm and aperture (opening) width of 5 μm.

[0146] (Buffer Layer 102)

[0147] With temperature set to 1050° C., a buffer layer 102 made ofAl_(0.25)Ga_(0.95)N is formed to a thickness of 4 μm on the nitridesemiconductor substrate, which has been formed as described above, byusing TMG (trimethyl gallium), TMA (trimethyl aluminum) and ammonia.This layer functions as a buffer layer between the n-type contact layermade of AlGaN and the nitride semiconductor substrate., Then layerswhich constitute the device structure are formed on the base layer madeof nitride semiconductor.

[0148] Specifically, in the case of a substrate made of GaN formed bylateral growth, generation of pits can be decreased by using the bufferlayer 102 made of nitride semiconductor Al_(a)Ga_(1-a)N (0<a≦1) whichhas thermal expansion coefficient smaller than that of GaN. Preferablythe buffer layer is formed on the laterally grown layer of nitridesemiconductor GaN. When the proportion of Al in the buffer layer 102 isin a range of 0<a<0.3, the buffer layer can be formed with goodcrystallinity. The buffer layer may be formed as an n-type contactlayer, or the buffer layer 102 and the n-type contact layer 104 formedthereon may be caused to perform buffering effect by forming the n-typecontact layer of the sane composition as that of the buffer layer afterforming the buffer layer 102. That is, as the buffer layer 102 of atleast one layer is provided between the nitride semiconductor substratewhich employed, lateral growth or a laterally grown layer formed thereonand the device structure, or between the active layer in the devicestructure and the laterally grown layer (substrate) or the laterallygrown layer (substrate) formed thereon, or more preferably between thelower cladding layer provided on the substrate side of the devicestructure and the laterally grown layer (substrate), generation of pitscan be decreased and the device characteristics ca be improved. Thebuffer layer is capable of improving the crystallinity when forming theactive layer, particularly the thick nitride semiconductor layer whichincludes In according to the present invention, it is preferable toprovide the buffer layer.

[0149] (n-type Contact Layer 103)

[0150] The n-type contact layer 103 made of Al_(0.05)Ga_(0.95)N dopedwith Si is formed to a thickness of 4 μm at a temperature of 1050° C. onthe buffer layer 102, which has been formed as described above, by usingTMG, TMA, ammonia, and silane gas used as an impurity gas.

[0151] (Crack Preventing Layer 104)

[0152] Then a crack preventing layer 104 made of In_(0.05)Ga_(0.95)N isformed to a thickness of 0.15 μm at a temperature of 800° C. by usingTMG, TMI (trimethyl indium), and ammonia. The crack preventing layer maybe omitted.

[0153] (n-type Cladding Layer 105)

[0154] After growing a layer A made of undoped Al_(0.05)Ga_(0.95)N to athickness of 25 Å is grown at a temperature of 1050° C. by using TMA,TMG and ammonia as the stock material gas, supply of TMA is stopped andsilane gas is used as the impurity gas, and a layer B made of GaN dopedwith Si in concentration of 5×10¹⁸/cm³ is formed to a thickness of 25 Å.This operation is repeated 200 times to stack the layer A and the layerB thereby to form the n-type cladding layer 106 made in multi-layeredfilm (super lattice structure) having a total thickness of 1 μm. At thistime, a difference in the refractive index which is sufficient for thecladding layer to function can be provided when the proportion of Al ofthe undoped AlGaN is in a range from 0.05 to 0.3.

[0155] (n-type Optical Guide Layer 106)

[0156] Then at a similar temperature, an n-type optical guide layer 106made of undoped GaN is formed to a thickness of 0.15 μm by using TMG andammonia as the stock material gas. The n-type optical guide layer 107may also be doped with an n-type impurity.

[0157] (Active Layer 107)

[0158] Then by setting the temperature to 800° C., a barrier layer (B)made of In_(0.05)Ga_(0.95)N doped with Si in a concentration of5×10^(18/cm) ³ is formed to a thickness of 140 Å by using TMI (trimethylindium), TMG and ammonia as the stock material gas and silane gas as theimpurity gas. Then the supply of silane gas is stopped and a well layer(W) made of undoped In_(0.1)Ga_(0.9)N is formed to a thickness of 25 Å,while stacking the barrier layer (B) and the well layer (W) in the orderof (B)/(W)/(B)/(W). Last, top barrier layer made of In_(0.05)Ga_(0.95)Nis formed to a thickness of 140 Å by using TMI (trimethyl indium), TMGand ammonia as the stock material gas. The active layer 107 becomesmultiple quantum well structure (MQW) having total thickness of 470 Å.

[0159] (p-type Electron Confinement Layer 108: First p-type NitrideSemiconductor Layer)

[0160] Then at a similar temperature, a p-type electron confinementlayer 108 made of Al_(0.3)Ga_(0.7)N doped with Mg in a concentration of1×10¹⁹/cm³ is formed to a thickness of 100 Å by using TMA, TMG andammonia as the stock material gas and Cp₂Mg (cyclopentadienyl magnesium)as the impurity gas. This layer may not be provided, though this wouldfunction as an electron confinement layer and help decrease thethreshold when provided. In this case, the p-type impurity Mg diffusesfrom the p-type electron confinement layer 108 into the top barrierlayer which is adjacent thereto so that the top barrier layer becomesdoped with Mg of about 5 to 10×10¹⁶/cm³.

[0161] (p-type Optical Guide Layer 109)

[0162] Then by setting the temperature to 1050° C., a p-type opticalguide layer 109 made of undoped GaN is formed to a thickness of 0.15 μmby using TMG and ammonia as the stock material gas. While the p-typeoptical guide layer 109 is grown as an undoped layer, diffusion of Mgfrom the adjacent layers such as the p-type electron confinement layer108 and the p-type cladding layer 109 increases the Mg concentration to5×10¹⁶/cm³ and turns the layer to p-type. Alternatively, this layer maybe intentionally doped with Mg while growing.

[0163] (p-type Cladding Layer 110)

[0164] Then a layer of undoped Al_(0.05)Ga_(0.95)N is formed to athickness of 25 Å at 1050° C., then supply of TMA is stopped and a layerof Mg-doped GaN is formed to a thickness of 25 Å by using Cp₂Mg. Thisoperation is repeated 90 times to form the p-type cladding layer 110constituted from super lattice structure of total thickness of 0.45 μm.When the p-type cladding layer is formed in super lattice structureconsisting of nitride semiconductor layers of different band gap energylevels with at least one nitride semiconductor layer including Al beingstacked one on another, crystallinity tends to be improved by doping oneof the layers more heavily than the other, in the so-called modulateddoping. In the present invention, however, both layers may be dopedsimilarly. The cladding layer 110 is made of nitride semiconductor whichincludes Al, preferably in super lattice structure which includesAl_(X)Ga_(1-X)N (0<X<1), more preferably super lattice structureconsisting of GaN and AlGaN stacked one on another. Since the p-typecladding layer 110 formed in the super lattice structure makes itpossible to increase the proportion of Al in the entire cladding layer,refractive index of the cladding layer can be decreased. Also becausethe band gap energy increases, it is very effective in reducing thethreshold value. Moreover, since pits generated in the cladding layercan be reduced by the super lattice structure compared to a case withoutsuper lattice structure, occurrence of short-circuiting is also reduced.

[0165] (p-type Contact Layer 111)

[0166] Last, at a temperature of 1050° C., a p-type contact layer Illmade of p-type GaN doped with Mg in a concentration of 1×10²⁰/cm³ isformed to a thickness of 150 Å on the p-type cladding layer 110. Thep-type contact layer 111 may be formed from p-typeIn_(x)Al_(y)Ga_(1-x-y)N (0≦X, 0≦Y, X+Y≦1), and preferably from Mg-dopedGaN which achieves the best ohmic contact with the p-type electrode 120.Since the contact layer 111 is the layer where the electrode is to beformed, it is desirable to have a high carrier concentration of1×10¹⁷/cm³ or higher. When the concentration is lower than 1×10¹⁷/cm³,it becomes difficult to achieve satisfactory ohmic contact with theelectrode. Forming the contact layer in a composition of GaN makes itcasier to achieve satisfactory ohmic contact with the electrode. Afterthe reaction has finished, the wafer is annealed in nitrogen atmosphereat 700° C. in the reaction vessel thereby to further decrease theelectrical resistance of the p-type layer.

[0167] After forming the nitride semiconductor layers one on another asdescribed above, the wafer is taken out of the reaction vessel. Then aprotective film of SiO₂ is formed on the surface of the top-most p-typecontact layer, and the surface of the n-type contact layer 103 whereonthe n-type electrode is to be formed is exposed as shown in FIG. 1 byetching with SiCl₄ gas in the RTEF (reactive ion etching) process. Forthe purpose of deep etching of the nitride semiconductor, SiO₂ is bestsuited as the protective film.

[0168] Then ridge stripe is formed as the striped waveguide regiondescribed above. First, a first protective film 161 having thickness of0.5 μm is formed from Si oxide (mainly SiO₂) over substantially theentire surface of the top-most p-type contact layer (upper contactlayer) by means of a PDP apparatus. Then the first protective film 161is patterned with stripe width of 1.6 μm with a mask of a predeterminedconfiguration being placed thereon by means of photolithography processand the RIE (reactive ion etching) apparatus which employs CF₄ gas. Atthis time, height of the ridge stripe (depth of etching) is set so thatthickness of the p-type optical guide layer 109 becomes 0.1 μm bypartially etching the p-type contact layer 111, the p-type claddinglayer 109 and the p-type optical guide layer 110.

[0169] After forming the ridge stripe, a second protective layer 162made of Zr oxide (mainly ZrO₂) is formed on the first protective layer161 to a thickness of 0.5 μm continuously over the first protectivelayer 161 and the p-type optical guide layer 109 which has been exposedby etching.

[0170] After forming the second protective film 162, the wafer issubjected to heat treatment at 600° C. When the second protective filmis formed from a material other than SiO₇, it is preferable to applyheat treatment at a temperature not lower than 300° C., preferably 400°C. or higher but below the decomposition temperature of the nitridesemiconductor (1200° C.) after forming the second protective film, whichmakes the second protective film less soluble to the material(hydrofluoric acid) that dissolves the first protective film, thus it isdesirable to add this process.

[0171] Then the wafer is dipped in hydrofluoric acid to remove the firstprotective film 161 by the lift-off process. Thus the first protectivefilm 161 provided on the p-type contact layer 111 is removed thereby toexpose the p-type contact layer. The second protective film 162 isformed on the side faces of the ridge stripe and the plane whichcontinues therefrom (exposed surface of the p-type optical guide layer109) as shown in FIG. 1.

[0172] After the first protective film 161 provided on the p-typecontact layer 112 is removed as described above, a p-type electrode 120made of Ni/Au is formed on the surface of the exposed p-type contactlayer 111 as shown in FIG. 1. The p-type electrode 120 is formed withstripe width of 100 μm over the second protective film 162 as shown inFIG. 1. After forming the second protective film 162, an n-typeelectrode 121 made of Ti/Al in stripe configuration is formed in adirection parallel to the stripe on the n-type contact layer 103 whichhas been already exposed.

[0173] Then the surface of a desired region which has been exposed byetching where lead-out electrodes for the p-type and n-type electrodesare to be formed is masked, and a multi-layered dielectric film 164 madeof SiO₂ and TiO₂ are formed. Lead-out electrodes 122, 123 made ofNi—Ti—Au (1000 Å-1000 Å-8000 Å) are formed on the p-type and n-typeelectrodes. At this time, the active layer 107 is formed with a width of200 μm (width in the direction perpendicular to the resonatordirection). The multi-layered dielectric film made of SiO₂ and TiO₂ areformed also on the resonator surface (reflector side).

[0174] After forming the n-type and p-type electrodes as describedabove, the wafer is divided into bar shape along M plane (M plane ofGaN, (11-00) or the like) of the nitride semiconductor in the directionperpendicular to the striped electrode. The wafer of bar shape isfurther divided to obtain laser devices with the resonator length being650 μm.

[0175] The laser device made as described above has the stakingstructure 20 shown in FIG. 7 and shows the band gap energy diagrams andcorresponds to the first, second, fourth and fifth embodiments. Whendividing the wafer into bars, the wafer may be cleaved along thewaveguide interposed between etched end faces with the cleavage surfacebeing used as the resonator surface, or may be cleaved at a positionother than the waveguide with the etched end faces being used as theresonator surface, or one of the etched end faces and a cleavage surfacemay be used as a pair of resonator surfaces. While a reflecting filmmade of a multi-layered dielectric film is provided on the resonatorsurface of the etched end faces, a reflecting film may also be providedon the resonator surface of the cleavage surface after cleaving. Thereflecting film may be made of at least one selected from among a groupconsisting of SiO₂, TiO₂, ZrO₂, ZnO, Al₂O₃, MgO and polyimide. Thereflecting film may also comprise multiple films each having a thicknessof λ/4n (λ is the wavelength and n is the refractive index) stacked oneon another, or may comprise a single layer, and may also be madefunction as a surface protective film which prevents the resonator endfaces from being exposed, as well as the reflecting film. To function asa surface protective film, the film may be made in a thickness of λ/2n.Such a laser device may also be made as only the n-electrode formingsurface (m-type contact layer) is exposed without forming the etchingend face in the device manufacturing process, and a pair of cleavagesurfaces are used as the resonator surfaces.

[0176] When dividing the bar-shaped wafer, too, cleavage surface of thenitride semiconductor (single substrate) may be used. Alternatively, thebar may be cleaved in M plane and A plane ({1010}), of the nitridesemiconductor (GaN) perpendicular to the cleavage surface which is madewhen cleaving into the bar, approximated by hexagonal system, thereby toobtain chips. Also the A plane of the nitride semiconductor may be usedwhen cleaving into bars.

[0177] A laser device capable of continuous oscillation at 405 nm withoutput power of 5 to 30 mW and threshold current density of 2.8 kA/cm²at the room temperature can be made. The laser device thus obtained haslifetime of 2000 to 3000 hours which is two to three times that ofComparative Example 1 operating with continuous oscillation with outputof 5 mW at 60° C. Reverse withstanding voltage of this device wascompared with that of Comparative Example 1. Many of the laser deviceswere not destroyed and when the voltage was raised to 100 V, some werenot destroyed showing the reverse withstanding voltage characteristicabout twice higher than that of Comparative Example 1.

EXAMPLE 2

[0178] Laser devices are made similarly to Example 1 except for thebarrier layers located in the interface between the active layer and thep-type electron confinement layer (last-stacked barrier layer andbarrier layer nearest to the p side), among the barrier layers providedin the active layer, are doped Mg in concentration of 1×10¹⁸/cm³. Thelaser device thus obtained has the last barrier layer doped with Mg moreheavily than in the case of Example 1, and has lifetime and reversewithstanding voltage characteristic of similar level.

EXAMPLE 3

[0179] Laser devices are made similarly to Example 1 except for theactive layer is formed as described below.

[0180] (Active Layer 107)

[0181] A barrier layer (B) made of In_(0.05)Ga_(0.95)N doped with Si ina concentration of 5×10¹⁸/cm³ is formed to a thickness of 140 Å at atemperature of 800° C. by using TMI (trimethyl indium), TMG and ammoniaas the stock material gas and silane gas as impurity gas. Then thesupply of silane gas is stopped and a well layer (W) made of undopedIn_(0.1)Ga_(0.9)N is formed to a thickness of 40 Å, while stacking thebarrier layer (B) and the well layer (W) in the order of(B)/(W)/(B)/(W). Last, the last barrier layer made of undopedIn_(0.05)Ga_(0.95)N is formed by using TMI (trimethyl irdium), TMG andammonia as the stock material gas. The active layer 107 becomes multiplequantum well structure (MQW) having total thickness of 500 Å.

[0182] A laser device capable of continuous oscillation at a wavelengthof 405 nm with output power of 5 to 30 mW and threshold current densityof 2.8 kA/cm² at the room temperature can be made. The laser device thusobtained has lifetime of 5000 to 6000 hours in operation of continuousoscillation with output of 5 mW at 60° C. Reverse withstanding voltageof this device was compared with that of Comparative Example 1. This isequivalent to a lifetime near 100 thousand hours at the roomtemperature. The reverse withstanding voltage is about 45V.

EXAMPLE 4

[0183] Laser devices are made similarly to Example 1 except that thebarrier layers in the interface between the active layer and the p-typeelectron confinement layer (last barrier layer), among the barrierlayers provided in the active layer, are doped Mg in concentration of1×10¹⁸/cm³. The laser device thus obtained has the last barrier layerdoped with Mg more heavily than in the case of Example 1, and haslifetime and reverse withstanding voltage characteristic of similarlevel.

EXAMPLE 5

[0184] In Example 1, thickness of the well layer is set to 55 Å. Thelaser device has lifetime significantly longer than that of Example 1,lasting for 1000 to 2000 hours in continuous oscillation with outputpower of 30 mW at 50° C.

[0185] When thickness of the well layer is increased to 60, 80 and 90 Åin Example 1, lifetime of the device rends to increase roughly inproportion to the thickness. At the same time, increases in the value ofVf and threshold current are observed as a result of the increase in thetotal thickness of the active layer as the well layer becomes thicker.In any of these cases, however, very long lifetime of the device isachieved in comparison to Comparative Example 1. With regards to Vf andthreshold current, while definite conclusion cannot be drawn becausethese characteristics are related to the total thickness of the activelayer and depend on the stacking structure, these characteristics do notdepend much on the change in the thickness of the well layer in case thenumber of well layers is 2, which is the least number of well layers inmultiple quantum well structure, and increases in the value of vf andthreshold current are kept at low levels, namely insignificant amountsof increase over Example 1, never resulting in a serious degradation ofthe device characteristics during continuous oscillation of the LD. Tosum up, device characteristics can be improved when the thickness of thewell layer is 40 Å or larger, and device lifetime can be increasedsignificantly when the thickness is 50 Å or larger. When the thicknessof the well layer is 50 Å or larger, oscillation with output power of 80mW can be achieved with some devices achieving an output of 100 mW.

EXAMPLE 6

[0186] When the thickness of the last barrier layer (barrier layerlocated at the topmost position) is increased to 150 Å in Example 1, adevice having lifetime longer than that of Example 1 was obtained. Thisis supposedly because, as shown in FIGS. 9A and 9B, the increasingthickness of the topmost barrier layer 2 c, which means increasingdistance d_(B) between the well layer 1 b and the p-type electronconfinement layer 28, keeps the well layer from the first p-type nitridesemiconductor layer (p-type electron confinement layer) that has a highresistance and is expected to be heated to a higher temperature than theother layers during operation of the device, thereby protecting the welllayer from the adverse effect of the high temperature and allows laseroscillation with better oscillation characteristics.

EXAMPLE 7

[0187] In Example 1, the active layer is made in such a structure asbarrier layer, well layer, barrier layer and well layer stacked in thisorder with the barrier layer having thickness of 70 Å, with a harrierlayer 140 Å thick provided at the end. Lifetime of the device whenoperated under conditions of continuous oscillation with output of 30 mWat 50° C. is shown in FIG. 12 with the thickness of the well layer beingchanged as 22.5 Å, 45 Å, 90 Å and 130 Å. As will be clear from thedrawing, the thicker the well layer becomes, the longer the lifetime ofthe device, thus making a laser device of longer lifetime. When thethickness of the well layer is 22.5 Å, 45 Å, 90 Å or 130 Å, oscillationof 30 mW or high output power can be made similarly to the case ofExample 5, and laser device having output power of 80 to 100 mW can beachieved when the thickness is 90 Å or 130 Å.

EXAMPLE 8

[0188] In Example 1, the active layer is made in such a structure asbarrier layer, well layer, barrier layer and well layer stacked in thisorder with the well layer having thickness of 45 Å, with a barrier layer140 Å thick provided at the end. Lifetime of the device when operatedunder conditions of continuous oscillation with output of 30 mW at 5° C.is shown in FIG. 13 with the thickness of the barrier layer other thanlast barrier being changed as 22.5 Å, 45 Å, 90 Å and 130 Å. As will beclear from the drawing, when the thickness of the barrier layer isincreased, device lifetime remains substantially constant with thethickness around 50 Å and larger. Thus satisfactory device lifetime canbe made of the nitride semiconductor device of the present inventionwhen the barrier layer 40 Å or higher.

EXAMPLE 9

[0189] Laser devices are obtained similarly to Example 1 except forsetting the proportion of Al in the AlGaN layer of the multi-layeredcladding layer to 0.1. The laser devices obtained have mean proportionof Al being 0.05 in the cladding layer, with self-excited oscillationbeing observed in some of them during continuous oscillation in thesingle mode with 30 mW. When the proportion of Al in the AlGaN layer ofthe multi-layered cladding layer to 0.15, mean proportion of Al in thecladding layer becomes 0.78, and the probability of self-excitedoscillation to occur becomes higher than in the case where meanproportion of Al is 0.05. Thus a laser device free of self-excitedoscillation can be obtained by setting the proportion of Al in thecladding layer to 0.05 or lower, preferably 0.025 or lower, or 0.03 orlower.

EXAMPLE 10

[0190] Laser devices are obtained similarly to Example 1 except forgrowing the topmost barrier layer (barrier layer disposed nearest to thep-type layer side) to a thickness of 150 Å. The laser device thusobtained shows a tendency of the device lifetime becoming slightlylonger than Example 1. On the contrary, the laser device having thetopmost barrier layer of 100 Å in thickness has lifetime significantlyshorter than that of Example 1.

EXAMPLE 11

[0191] Laser devices are obtained similarly to Example 1 except forproviding the p-type optical guide layer 109 directly on the activelayer 107 without providing the p-type electron confinement layer 108.The laser device thus obtained has the value of Vf about 1V lower,although the threshold current increases sharply and some of the laserdevices are difficult to oscillate. This is supposedly because theabsence of the first p-type nitride semiconductor layer (p-type electronconfinement layer 108) of high resistance decreases the value of Vf andmakes it difficult to confine electrons in the active layer, thusleading to sharp increase in the threshold.

EXAMPLE 12

[0192] Laser devices are obtained similarly to Example 1 except formaking the active layer in a stacked structure of three well layers andfour barrier layers. The laser device thus obtained has the value of Vfhigher than that of Example 1 because the active layer becomes thickeras a whole, and the threshold current also becomes slightly higher dueto the larger number of the well layers. When the active layer is madeby stacking five barriers and four well layers alternately with abarrier layer at the end, the threshold current the value of Vf becomehigher than in the case where the number of well layers is 2 or 3.

[0193] In FIG. 4, when the first barrier layer (second n side barrierlayer) 2 a and the last barrier layer (first p side barrier layer) 2 dare grown to thickness of 140 Å and the barrier layers 2 b, 2 c aregrown to thickness of 100 Å (structure of the active layer shown in FIG.5), variations in the device characteristics, particularly variations inthe lifetime among the chips become less than in the case shown in FIG.5, thus providing laser devices of better device characteristics.

COMPARATIVE EXAMPLE 1

[0194] Laser devices are obtained similarly to Example 1 except that allbarrier layers in the active layer are doped with Si. The laser devicesthus obtained have lifetime of 1000 hours in continuous oscillation withoutput power of 5 mw at 60° C. In the evaluation of the reversewithstanding voltage, most of the Laser devices thus obtained aredestroyed when subjected to reverse voltage of 50V. Device lifetime andreverse withstanding voltage of the devices having the last barrierlayer in the active layer being doped with Si in concentrations of1×10¹⁷/cm³, 1×10¹⁸/cm³ and 1×10¹⁹/cm3 are shown in FIGS. 14, 15. In thegraph, the plot noted as undoped corresponds to the data of Example 1.As will be clear from the graphs, doping the barrier layer disposednearest to the p-type layer side in the active layer with Si causes thedevice lifetime and reverse withstanding voltage to decrease, resultingin degradation of the device characteristic in proportion to theconcentration of doping.

[0195] In an analysis of the laser devices thus obtained with SIMS(secondary ion mass spectroscopy analysis), Si and Mg are detected inthe topmost barrier layer disposed in the interface with the p-typeelectron confinement layer among the barrier layers in the active layer(barrier layer located at a position nearest to the p-type layer). Thusthe laser devices obtained has the topmost barrier layer doped with Siand Mg, which is believed to be the cause of the significantly lowercharacteristics than the laser device obtained in Example 1. However, asshown in FIGS. 14, 15, concentration of Mg doping does not change whenthe concentration of Si doping is changed, the degradation in the devicecharacteristics is supposedly caused mainly by n-type impurity.

EXAMPLE 13

[0196] In Example 1, laser devices are obtained by using an active layer407 described below with reference to FIG. 10 instead of the activelayer 107.

[0197] (Active Layer 407)

[0198] By setting the temperature to 880° C., a first barrier layer 401a made of In_(0.01)Ga_(0.99)N doped with Si in a concentration of5×10¹⁸/cm³ is formed to a thickness of 100 Å by using TMI, TMG andammonia as the stock material gas and silane gas as the impurity gas.Then with the temperature lowered to 820° C., the supply of silane gasis stopped and a well layer 402 a made of undoped Ino_(0.3)Ga_(0.7)N isformed to a thickness of 50 Å. At the same temperature, a second barrierlayer 403 a made of undoped Al_(0.3)Ga_(0.7)N is formed to a thicknessof 10 Å using TMA. The 3-layer structure of the first barrier layer 401a, the well layer 402 a and the second barrier layer 403 a is repeatedso as to stack layers 401 b, 402 b and 403 b, with a topmost barrierlayer 404 made of undoped In_(0.01)Ga_(0.99)N being formed to athickness of 140 Å at the end, thereby forming an active Layer 407 ofmultiple quantum well structure (MQW) having total thickness of 460 Å.At this time, p-type impurity Mg diffuses from the adjacent p-typeelectron confinement layer 108 into the topmost barrier layer 404located at a position nearest to the p-type layer, thus making thebarrier layer which includes Mg. Thus a laser device having high outputpower and long lifetime and emitting light of wavelength 470 nm isobtained. At this time, the second barrier layer provided on top of thewell layer is made of a nitride semiconductor which includes Al,preferably a nitride semiconductor having a composition ofAl_(z)Ga_(1-z)N (0<z<1), which is supposed to have an effect of formingproper unevenness in the well layer and cause segregation of In ordistribution in concentration, thus resulting in quantum dot or quantumwire, thereby providing a nitride semiconductor device having higheroutput power than the case without the second barrier layer. At thistime, proper unevenness tends to be formed in the well layer when theproportion z of Al is not less than 0.3. Similar effect can be achievedwhen the second barrier layer is provided not in contact with the welllayer. It is also preferable that the barrier layer provided below thewell layer in contact therewith does not include Al as in the case ofthe first barrier layer, because this enables it to form the well layerwith good crystallinity.

EXAMPLE 14

[0199] A light emitting device shown in FIG. 9A and 9B is manufacturedas follows.

[0200] A substrate made of sapphire (C plane) is set in aMOVPE reactionvessel, with the substrate temperature raised to 1050° C. while flowinghydrogen, and the substrate is cleaned.

[0201] (Buffer Layer 302)

[0202] With the temperature lowered to 510° C., a buffer layer 302 madeof GaN is formed to a thickness of 150 Å on the substrate 301 by usinghydrogen as the carrier gas, and ammonia, TMG (trimethyl gallium), TMA(trlmethyl aluminum) as the stock material gas.

[0203] (Base Layer 303)

[0204] After growing the buffer layer 302, supply of only the TMG isstopped and the temperature is raised to 1050° C. At the temperature of1050° C., a base layer 303 made of undoped GaN is grown to a thicknessof 1.5 μm using TMG and ammonia gas as the stock material gas. The baselayer serves as the substrate whereon to grow the nitride semiconductor.

[0205] (n-type Contact Layer 304)

[0206] Then an n-type contact layer 304 made of GaN doped with Si inconcentration of 4.5×10¹⁸/cm³ is formed to a thickness of 2.25 μm at atemperature of 1050° C. by using TMG and ammonia as the stock materialgas, and silane gas as an impurity gas.

[0207] (n-type First Multi-layered Film Layer 305)

[0208] Then with supply of only the silane gas stopped, abase layer 305a made of undoped GaN is formed to a thickness of 3.000 Å at atemperature of 1050° C. using TMG and ammonia gas, followed by thegrowth of an intermediate layer 305 b made of GaN doped with Si inconcentration of 4.5×10¹⁸/cm³ to a thickness of 300 Å at the sametemperature by adding the silane gas. Then again with supply of only thesilane gas stopped, an upper layer 305 c made of undoped GaN is formedto a thickness of 50 Å at the same temperature, thereby forming a firstmulti-layered film layer 305 consisting of three layers 304 a, 305 b and.304 c with total thickness of 3350 Å.

[0209] (n-type Second Multi-layered Film Layer 306)

[0210] Then at roughly the same temperature, the second nitridesemiconductor layer made of undoped GaN is grown to a thickness of 40 Å,followed by the growth of the first nitride semiconductor layer made ofundoped In_(0.13)Ga_(0.87)N to a thickness of 20 Å at 800° C. using TMG,TMI and ammonia. This operation is repeated to stack the second nitridesemiconductor layer and the first nitride semiconductor layeralternately ten times in this order. Last, the second nitridesemiconductor layer made of GaN is grown to a thickness of 40 Å, therebyforming the n-type second multi-layered film layer 306 of super latticestructure having thickness of 640 Å.

[0211] (Active Layer 307)

[0212] A barrier Layer made of GaN is grown to a thickness of 250 Å,followed by the growth of well layer made of undoped In_(0.3)Ga_(0.7)Nto a thickness of 30 Å at 800° C. using TMG, TMI and ammonia. Bystacking seven barrier layers and six well layers alternately in such aconstitution as barrier layer B₁/well layer/barrier layer B₂welllayer/barrier layer B₃/well layer/barrier layer B/well layer/barrierlayer B₅/well layer/barrier layer B₆/well layer/barrier layer B₇,thereby forming the active layer 307 of multi-quantum well structurehaving total thickness of 1930 Å. At this time, the barrier layers B₁,B₂ are doped with Si in concentration of 1×10¹⁷/cm3 and the otherbarrier layers B. (i=3, 4, . . . ,7) are grown undoped.

[0213] (p-type Multi-layered Cladding Layer 308)

[0214] A third nitride semiconductor layer made of Al_(0.2)Ga_(0.8)Ndoped with Mg in a concentration of 1×10²⁰/cm³ is formed to a thicknessof 40 Å at 1050° C. by using TMG, TMA, ammonia and Cp₂Mg(cyclopentadienyl magnesium) as the impurity gas. Then with thetemperature being set to 800° C., a fourth nitride semiconductor layermade of In_(0.03)Ga_(0.97)N doped with Mg in a concentration of1×10²⁰/cm³ is grown to a thickness of 25 Å by using TMG, TMI, ammoniaand Cp₂Mg. These operations are repeated to stack the third nitridesemiconductor layer and the fourth nitride semiconductor layeralternately five times in this order, with one layer of the thirdnitride semiconductor layer having thickness of 40 Å being grown at theend. Thus the p-type multi-layered cladding layer 308 of super latticestructure having thickness of 365 Å is formed.

[0215] (p-type GaN Contact Layer 310)

[0216] Then with the temperature being set to 1050° C., a p-type contactlayer 310 made of p-type GaN doped with Mg in a concentration of1×10²⁰/cm³ is grown to a thickness of 700 Å by using TMG, ammonia andCp₂Mg.

[0217] Upon completion of the reaction, the temperature is lowered tothe room temperature, and the wafer is annealed at 700° C. in nitrogenatmosphere within the reaction vessel, thereby to decrease theresistance of the p-type layer.

[0218] After annealing, the wafer is taken out of the reaction vessel. Amask of a predetermined shape is formed on the surface of the p-typecontact layer 310 provided on the top, and the p-type contact layer isetched in an RIE (reactive ion etching) apparatus, thereby exposing then-type contact layer 4 as shown in FIG. 9A and 9B.

[0219] After etching, a p-type electrode 311 which transmits light madeof a material including Ni and Au is formed to a thickness of 200 Å oversubstantially the entire surface of the p-type contact layer 310provided on the top, and a p-type pad electrode made of Au for bondingis formed to a thickness of 0.5 μm on the p-type electrode 11. On theother hand, an n-type electrode 312 which includes W and Al is formed onthe surface of the n-type contact layer 304 which has been exposed byetching, thereby to obtain a light emitting device. In the lightemitting device-thus obtained, since the barrier layer B₁ which islocated nearest to the n-type layer and the next barrier layer B₂ aredoped with an n-type impurity, the carrier from the n-type layer isefficiently injected deep into the active layer (toward the p-typelayer), thereby improving the photo-electric conversion efficiency,decreasing the value, of Vf and the leak current, and increasing thelight emission output power compared to Comparative Example 2 whereinall the barrier layers are grown undoped.

EXAMPLE 15

[0220] Light emitting devices are obtained similarly to the Example 14except that the barrier layer B₇ located nearest to the p-type layer isdoped with Mg as a p-type impurity in concentration of 1×10¹⁸/cm³ in theExample 14. In the light emitting device thus obtained, since thetopmost barrier layer B₇ has a p-type impurity, the carrier from thep-type layer is efficiently injected, thereby improving thephoto-electric conversion efficiency, and increasing the light emissionoutput power compared to Example 14.

COMPARATIVE EXAMPLE 2

[0221] Light emitting devices are obtained similarly to Example 14except that all the barrier layers and the well layers in the activelayer are grown undoped in Example 14. The light emitting devices thusobtained have lower light emission output power and shorter lifetimethan those of Example 14.

EXAMPLE 16

[0222] A plane emission type laser device shown in FIG. 11 will bedescribed below.

[0223] (Substrate 501)

[0224] A substrate 501 similar to the nitride semiconductor substrate101 used in Example 1 is used.

[0225] A reflecting film 530 is formed by stacking a first layer 531made of AiN and a second layer 532 made of GaN alternately three timeson the nitride semiconductor substrate 501. Each of the stacked layersis formed to a thickness of λ/4n (λ is the wavelength and n is therefractive index) wherein n is set to 2 (AlN) or 2.5 (GaN) with thethickness being about 500 Å for the first layer and 400 Å for the secondlayer. At this time, the reflecting film may comprise the first and thesecond layers made of nitride semiconductor represented byIn_(x)Al_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1, x+y=1), while the first and thesecond layers of the reflecting film made of nitride semiconductor arepreferably multi-layered film formed by stacking nitride semiconductorsof different compositions represented by Al_(x)Ga_(1-x)N (0≦x≦1). Atthis time, each layer is formed once or more, and one or more pairs ofthe first layer and the second layer are formed. Specifically, the pairof the first layer and the second layer may be formed in such aconstitution as AlGaN/AlGaN, GaN/AlGaN, AlGaN/AlN or GaN/AlN.Composition of Al_(x)Ga_(1-y)N/Al_(y)Ga_(1-y)N (0<x, x<y<1) is amulti-layered film of AlGaN and is therefore capable of decreasing thedifference in the thermal expansion coefficient and achieve goodcrystallinity. Composition of GaN/Al_(y)Ga_(1-y)N (0<y<1) makes itpossible to form a multi-layered film of GaN layer having improvedcrystallinity. When the difference in the proportion of Al (y-x) isincreased, difference in the refractive index between the first layerand the second layer increases resulting in a higher reflectivity.Specifically, a multi-layered reflecting film of high reflectivity canbe made by setting y-x≧0.3, preferably y-x≧0.5. Also similarly toExample 1, the multi-layered film made of Al_(y)Ga_(1-y)N (0<y≦1)functions as the buffer layer 102 and provides an effect of decreasingpits.

[0226] Then an n-type contact layer 533, an active layer 534, a p-typeelectron confinement layer (not shown) and a p-type contact layer 535are stacked under conditions similar to those of Example 2 (well layer55 Å), and a blocking layer 536 made of SiO₂ having a circular apertureis provided. A second p-type contact layer 537 is formed by growingMg-doped GaN through the circular aperture. At this time, either one ofthe p-type contact layer 535 and the second p-type contact layer 537 maybe formed. A multi-layered dielectric film made of SiO₂/TiO₂ is formedon the second p-type contact layer 537 to make a reflecting film 538,which is provided on the aperture of the blocking layer 536 in acircular configuration. Then the p-type contact layer 535 is exposed byetching, and a ring-shaped r-type electrode 421 is formed on the p-typecontact layer 535 which has been exposed and a p-type, electrode 520which surrounds the reflecting film 538 is formed on the second p-typecontact layer 537. The planar emission type laser device thus obtainedhas excellent device lifetime and high output power similar to that ofExample 2.

EXAMPLE 17

[0227] Laser devices are obtained similarly to Example 1 except that theactive layer and the p-type cladding layer are formed as describedbelow.

[0228] (Active Layer 107)

[0229] A barrier layer (B) made of In_(0.05)Ga_(0.95)N doped with Si ina concentration of 5×10¹⁸/cm³ is formed to a thickness of 70 Å. Then thesupply of silane gas is stopped and a well layer (W) made of undopedIn_(0.15)Ga_(0.9)N is formed to a Thickness of 70 Å, while stacking thebarrier layer (B) and the well layer (W) in the order of (B)/(W)(B)/(W). Last, topmost barrier layer made of undoped In_(0.05)Ga_(0.95)Nis formed to a thickness of 150 Å by using TMI (trimethyl indium) as thestock material gas. The active layer 107 becomes multiple quantum wellstructure (MQW) having total thickness of 430 Å.

[0230] (p-type Cladding Layer 110)

[0231] Then layer of undoped Al_(0.10)Ga_(0.95)N is formed to athickness of 25 Å and a layer of Mg-doped GaN is formed to a thicknessof 25 Å. This operation is repeated 90 times to form the p-type claddinglayer 110 constituted from super lattice structure of total thickness of0.45 μm.

[0232] In the laser device obtained as described above, ratio Rt of thebarrier except for the barrier layer located nearest to the p-type layerin the active layer and the well layer is 1, satisfying the relationshipbetween the well layer thickness and the device lifetime shown in FIG.12. Thus a laser device having high output power and long lifetime canbe provided. Also as the thickness of the barrier layer (the n sidebarrier layer or the barrier layer sandwiched by the well layers) isdecreased, a laser device having excellent response characteristic andRIN is provided for an optical disk system. Also as the proportion of Alin the p-type cladding layer is increased, difference in the refractiveindex thereof from that of the buried layer 162 becomes smaller,resulting in a laser device of effective refractive index type havingless confinement in the lateral direction which is free from kink evenin a high output region. It is preferable to provide the p-type claddinglayer which has the mean composition of Al_(x)Ga_(1-x)N in meanproportion x being in a range of 0<x≦0.1. This suppresses the kinkfromation in the laser device.

[0233] According to the present invention, a nitride semiconductordevice having excellent device lifetime and high output power can beobtained where the reverse withstanding voltage, of which weakcharacteristic of a device made of nitride semiconductor has been aproblem in the prior art, is improved. When the nitride semiconductordevice of the present invention is applied to laser device, a laserdevice having the improved characteristics similarly to those describedabove and is free from self-excited oscillation can be obtained.

What is claimed is:
 1. A nitride semiconductor device having a structurewherein an active layer of a quantum well structure, which has a welllayer made of a nitride semiconductor that includes In and a barrierlayer made of a nitride semiconductor, is sandwiched between a p-typenitride semiconductor layer and an n-type nitride semiconductor layer,wherein said active layer has, as said barrier layer, a first barrierlayer arranged in a position nearest to said p-type nitridesemiconductor layer and a second barrier layer that is different fromthe first barrier layer; and wherein said first barrier layer does notsubstantially include an n-type impurity while said second barrier layerincludes an n-type impurity.
 2. The nitride semiconductor deviceaccording to claim 1, wherein the film thickness of said first barrierlayer is greater than the film thickness of said second barrier layer.3. The nitride semiconductor device according to claim 1, wherein saidactive layer has L (L≧2) barrier layers so that the barrier layerarranged in a position nearest to said n-type nitride semiconductorlayer is denoted as barrier layer B1 and the i-th barrier layer (i=1, 2,3, . . . L) counted from the barrier layer B1 toward said p-type nitridesemiconductor layer is denoted as barrier layer Bi; and barrier layersBi from i=1 to i=n (1<n<L) include an n-type impurity.
 4. The nitridesemiconductor device according to claim 1, wherein the entire barrierlayers other than said first barrier layer include an n-type impurity.5. The nitride semiconductor device according to claim 1, wherein saidfirst barrier layer is arranged in the outermost position in said activelayer.
 6. The nitride semiconductor device according to claim 7, whereinsaid second barrier layer is arranged in the outermost position close tosaid n-type nitride semiconductor layer within said active layer.
 7. Thenitride semiconductor device according to claim 6, wherein the filmthickness of said first barrier layer is approximately the same as thefilm thickness of said second barrier layer.
 8. The nitridesemiconductor device according to claim 7, wherein said active layer has2 or more well layers and has a third barrier layer between the welllayers; and the film thickness of said third barrier layer is smallerthan the film thickness of said first p side barrier layer and saidsecond n side barrier layer.
 9. The nitride semiconductor deviceaccording to claim 1, wherein a: least one well layer within said activelayer has a film thickness of not less than 40 Å.
 10. The nitridesemiconductor device according to claim 1, wherein said first barrierlayer has a p-type impurity.
 11. The nitride semiconductor deviceaccording to claim 1, wherein said first barrier layer includes a p-typeimpurity in the range, of no less than 5×10¹⁶ cm⁻³ and no more than1×10¹⁹ cm⁻³.
 12. The nitride semiconductor device according to claim 1,wherein said first barrier layer is p-type or i-type.
 13. The nitridesemiconductor device according to claim 12, wherein said first barrierlayer has been grown without being doped with an impurity and includes ap-type impurity through diffusion from said p-type nitride semiconductorlayer.
 14. The nitride semiconductor device according to claim 1,wherein said n-type nitride semiconductor layer, said active layer andsaid p-type nitride semiconductor layer are layered in sequence.
 15. Thenitride semiconductor device according to claim 1, wherein said p-typenitride semiconductor layer has an upper clad layer made of a nitridesemiconductor that includes Al of which the average mixed crystal ratiox is in the range of 0<x≦0.05; said n-type nitride semiconductor layerhas a lower clad layer made of a nitride semiconductor that includes Alof which the average mixed crystal ratio x is in the range of 0<x≦0.05;and the nitride semiconductor device has a laser device structure. 16.The nitride semiconductor device according to claim 1, wherein saiddevice has a first p-type nitride semiconductor layer adjoining theactive layer in said p-type nitride semiconductor layer, and said firstp-type nitride semiconductor layer is made of a nitride semiconductorthat includes Al.
 17. The nitride semiconductor device according toclaim 16, wherein said first p-type nitride semiconductor layer isprovided so as to contact a barrier layer nearest to said p-type nitridesemiconductor layer and has been grown being doped with a p-typeimpurity of which concentration is higher than that of said barrierlayer in said active layer.
 18. The nitride semiconductor deviceaccording to claim 1, wherein the number of well layers in said activelayer is from 1 to
 3. 19. The nitride semiconductor device according toclaim 1, in said active layer said second barrier layer is arrangedbetween well layers and the film thickness ratio Rt (=[film thickness ofa well layer]/[film thickness of a barrier layer]) of said well layer tothe second barrier layer is in the range of 0.5≦Rt≦3.
 20. The nitridesemiconductor device according to claim 1, wherein the film thickness dwof said well layer is an the range of 40 Å≦dw≦100 Å while the filmthickness db of said second barrier layer is in the range of db≦40 Å.21. The nitride semiconductor device according to claim 1, wherein saidp-type nitride semiconductor layer has an upper clad layer made of anitride semiconductor that includes Al and said n-type nitridesemiconductor layer has a lower clad layer made of a nitridesemiconductor, wherein the average mixed crystal ratio of Al in theupper clad layer is greater than that of the lower clad layer.
 22. Thenitride semiconductor device according to claim 21, wherein the averagemixed crystal ratio x of Al in said upper clad layer is in the range of0<x≦0.1.
 23. The nitride semiconductor device according to claim 1,wherein said p-type nitride semiconductor layer has a first p-typenitride semiconductor layer which contacts said active layer and becomesan electron confining layer; said active layer has a well laycr of whichdistance dB from the first p-type nitride semiconductor layer is in therange of no less than 100 Å and no more than 400 Å and has a firstbarrier layer within the distance dB.